Welcome to The T-Dub Club!
Head on over to this thread and get the manuals.
You should find just about everything you need to know in those documents, get familiar with them, they will save you a ton of money, time and aggravation.
Next go to the Tech section of this forum and "Read" all of the pinned topics. (Don't just look at the pretty TW pictures, I know it's hard)
If you have specific questions start with a search on the forum, chances are it has been covered before.....usually over and over and over and over....well you get it. If you can't find what your looking for ask up! Someone here will point you in the right direction. There is a wealth of info on the board and inside the heads of the members and they enjoy sharing it. That is why we have such a great forum.
There are many websites to order OEM and after market parts. Shop around cause sometime shipping can kill a good deal. I like stadiumyamaha, boats.net and powersportsplus to compare OEM prices. A forum member works for procycle and has set up a TW specific page with a lot good stuff all in one place. Here is a link to an older thread with a bunch of parts links.
Lastly hurry up and order a shirt the deadline is approaching and this offer may not happen again.
Almost forgot. The 1987 have a different electric system and I'm not sure of the specifics. Most all parts are the same for all years with some exceptions and I don't know them all. Search the forum. It has been covered numerous times.
Some examples... Older bikes have front drum brakes where as newer bikes have a disc. Some have kick starters other don't. Some have fork oil drains the newer ones don't. Different carbs, different sprockets, different cam chain adjusters. But by and large the big parts are all the same. The biggest problem I'm aware of is the 87 CDI are prone to fail and they seem to be hard to find and $$$ cost a bunch of $$$