I agree with RobinMask. I think that ideally, learning to speak the language before you try to write is the most "natural" way to learn a language. Learning how to write so early on might get you hung up on... the writing portion of the language so much and so long that it will take away from your ability to communicate, which is what I think most people are looking for when they learn a language. Similarly, even if you are interested specifically in learning how to read a language, learning how to speak it to a degree first will probably help your reading comprehension quite a bit.
However, while speaking can get you a really long way in a language (I think that maybe a century ago a lot of people didn't know how to read), learning to read can really help you learn vocabulary (as Kyle said). However, without the basics that give you solid speaking, the use of that new vocabulary will be confusing. Knowing how to speak will also help to read kanji that you might already know the words for.
Learning from a textbook is starting from the "written" perspective on a language (usually... sometimes it can be some kind of hybrid). While textbooks do provide good examples of "polite & safe" language, they are hardly realistic in my experience. They will give you quick results though, which is something that immersion fails at in my opinion.
I'd say decide how serious you are about the language and set some kind of goal and practice schedule. Watch a bunch of Japanese TV and take a look at a lot of written sources (of both stuff you're interested in as well as some stuff that you might not be as interested). Just take a look at it and decide if you really want to do it. I think if you're serious about it, it will be about a 10 year commitment. If you're 17 right now, you might be entering the "real world" soon... the reality of which means a lot less time to study (which was ironic for me because when I had time to study I couldn't study what I wanted because I was too busy studying stuff that I didn't even care to study to begin with!
).