When it comes to reading VN's in Japanese, required skills can be grouped into four areas: Vocab, grammar, basic parsing skill, and kanji skill. In this post, for each area I'm going to explain:
-what knowing skills in the area are good for
-how you might study them
-how much you'll need to start reading.
I'll also give some related tips.
The requirements mentioned below are a conservative estimate. I've known people who've jumped in to playing VN's with less or much less, but I'm giving a safe estimate. A level which at most people, without any special knack for learning languages through immersion, should be able to gain traction. If you learn this much before starting an easyish VN, the amount you are completely lost should be significantly less than the percentage you are able to pick up and improve from.
This is not a comprehensive how-to guide by any means. Just an informative post.
Knowing enough vocab to study your grammar resource without being bogged down by vocab:
-About 30 verbs and 50 other words for Genki 1/ Tae kim Basic.
-By the time you get to Genki 2/Tae Kim Essential you'll want a good set of verbs (about 100), and maybe about 300 total vocabulary.
-~600 words about how much you'll want to be able to study N3 grammar without getting bogged down in vocab.
Having enough vocab to start your first VN:
-I recommend over 1000, but anywhere from 800-1300 is good. I remember trying Clannad with only 800, and I felt like ramming my head into a wall. It's also important to pick an easy title. It will still feel hard no matter what, but an easy title will be much more helpful and rewarding to play. You also must just translation aggregator and ITH. They are the reason why Visual Novels are the best medium for learning Japanese out of anime/books/movies/drama/etc.
There's a dedicated verb list here: http://nihongoichiban.com/2012/08/13/list-of-all-verbs-for-the-jlpt-n4/
Verbs are helpful to learn, because they are often the most important part of the sentence AND you need to to have stuff to conjugate.
In general JLPT-based vocab list is here: http://www.tanos.co.uk/jlpt/jlpt5/vocab/
Regarding English definitions:
Be mentally open and flexible. If the english definition doesn't quite add up, don't try and think about it too hard. Focus instead on associating the word with the situations where you see it.
For example, you might be confused by the word 都合 and it's unhelpful definition J-E definition, but if you seen 都合がいい used in a situation where you know it means "is convenient for me" from context then remember that occurence. There might be (there are, in fact), other usages of the word 都合, but that doesn't hurt you in anyway. The next time you see 都合 you can pair it against this meaning and see if that makes any sense.
2. Grammar Skills
With N5+N4 grammar you will be barely able to start making your way through a VN. Without N4, you will have quite limited gains in the long term from reading visual novels. (Equivalent to Genki 1+2.)
-Required to be able to play VN's
With N3 grammar, everything will feel a lot clearer, the amount of grammar you'll understand will exceed 60%. (Equiv. to Intermediate approach to Integrated Japanese). Highly recommended to study this before or soon after you start your first VN.
N2 grammar further cuts the amount of unknown grammar you face in three.
N1 is kind of like a bonus that gives you a lot of uncommon or formal expressions. It's NOT comprehensive at all, in terms of covered all Japanese phrases. From my experience, some of the phrases you learn in here show up often in novels (ばかり、んばかり), others quite less. Good to know, though
Expressions not covered in JLPT
There are a lot of patterns and phrases not covered in JLPT that you will see in typical native reading material. Examples (社長に議長, phrases like なんだと！？ Xってなんだ？ ですって！？ ～てくれないかな。 オレって、なんてバカなんだ ) Not to worry, many of them can be picked up as you go. For the rest, once you get settled into reading, you can start noting down those phrases you don't get and google them or ask other people.
Imabi for grammar
You can also try studying from http://www.imabi.net/. It's a phenomenal reference, it's just goes into tons of depth, too much. I think there's 2 or 3 times as much information there is covered by JLPT up to JLPT 1. As such it's going to be overwhelming for a beginner and is much better suited as a reference for intermediate or advanced learners.
3. Basic Parsing Skill
Knowing the different types of words (Covered by doing a vocab list of about 100 verbs, and then the JLPT 5 list. You also have to have done or be doing Tae Kim's Basic Guide, since he explains what na-adj's, i-adj's, and other word types are, etc.).
-(nouns, suru-verbs/nouns, verbs, na-adj's, i-adj's, adverbs, temporal adverbs)
-Required to be able to play VN's.
Knowing the basic sentence structure and how words can modify each other and fit in a sentence.: (adjectives modifying nouns, verbs).
The knowledge is covered by Tae Kim Basic + a mix of Essential Grammar and Genki 1/2. I personally find Tae Kim's explanation good even though the learning curve is steep and his lessons aren't good for review like Genki books are. He tries to convey to you the big picture.
-Required to be able to play VN's.
Being able to breakdown sentences and spot the different types of words based on their position.
-you can practice this by reading bits of text in your genki textbook, but more likely, the first time you really gain this skill is going to be the first month in which you read a visual novel with TA. Heavily practiced during your first month or two of reading VN's.
4. Kanji Skill:
Learning to spots radicals in kanji (could be covered by doing the 214 radicals, about 1 month. You could also do this ongoing basis, learning how to spot the radicals that make up a kanji, for the words you learn.)
-not needed to read VN's with TL aggregator, but extremely helpful for learning new words which have new kanji.
Learning to remember kanji, ie. start recognizing when words share the same kanji. (it is a long ongoing gradual process. You can start doing this with the vocab you learn once you are comfortable learning vocab. You can also pick out words you see in vn's and check whether they use the same kanji by typing them out (example 朝（あさ） and 朝食（ちょうしょく） use the same kanji.). Oh course, to be able to easily produce the kanji you want to compare you need to remember how to spell a word that contains it (in this case 朝). So, as your vocab expands, you'll be able to compare more kanji. Note that to be able to do this comparing you must be able to spot radicals in kanji (previous level skill).
-moderately helpful for learning vocabs. The same way remembering radicals helps learning with kanji: if you know the kanji clearly, you can remember a word just by the two kanji it uses, which is very precise and doesn't take a lot of mental bandwidth. It also means that you will much more rarely confuse words which have similiar looking kanji.
The following two skills are for more advanced, they won't be particularly useful until much later. You might not notice the problems they solve until later as well. I include them mainly for completeness.
Learning on-yomi for many of the Jyouyou kanji (start when you are intermediate-advanced, a medium-long process)
-helpful for exactly what it is, reading kanji words and compounds correctly.
-don't need to worry about this. From learning vocab you might pick up some of the common ones, but there's no need to pursue this actively for a while.
Learning kanji meaning: (start when you are advanced, and can use a J-J dictionary)
-suffixes like 府、省、性、症, as well normal kanji whose different meanings apply to clusters of words.
-helpful for kanji compounds which won't directly show up in dictionaries
-helpful for developing a native level understanding of vocabulary (not everything can be learned by exposure). A lot of literary words are fairly influenced by their kanji meanings, though sometimes consulting the word differentiation explanations can be more helpful.
One last topic...
On learning enough grammar and jumping into works too difficult for you.
Reading a VN isn't the best way to learn basic sentence structure. However, it's a great way to reinforce grammar points you've learned. It's also a great way to get an understanding of conversational patterns you won't find in textbooks or JLPT. But you won't have the presence of mind to pay attention to that if you are bogged down by not knowing basic grammar.
There are benefits for venturing early into native material or difficult vn's, but you wouldn't give a grade two student Tolkien, or even Harry Potter to improve their English. All the fancy prose and unusual concept would distract you from the more immediately useful things like, say: basic sentence structure.
There are works which are the right level, and there are VN's which you really want to read. For the best experience, it's best to find some combination of the two.
Ok that's all for now. Feel free to ask any questions: I didn't really go into the details of how to study, instead focusing on the, well, skills involved. But it's also hard to remember what it's like for someone just starting out. I remember parts of studying very clearly, but I forget the thousands of things I used to be puzzled through varying stages of understanding but now take for granted.
The process was all I could think about for the longest time. Now I don't give it much thought, it's just a regular part of my life, reading and a bit of studying. It's not bad idea, to just find a type of study that you know is helpful, stop thinking about all the right ways and wrong ways and magic tricks which don't exist, and just do it, for a while. Regularly. For a month or three.