You don't really need to waste your time on the entire alphabet, because once the knowledge of letter forms is there, it won't matter what letters you are using because the same principal is applied to each letter. Anyway start simple, keep your proportions consistent. You really want your painting abilities to reflect what you can do in a book, because if you have a dope blackbook but you can't paint for shit, then what is the point? Ideally you want to be able to not rely on a sketch; where you can just go to the wall and freestyle, because by then it will be like riding a bike. And you will know exactly what to do without having to think twice about it.