The first and foremost thing everyone should have for their ATV is a good shop manual. It will walk you step by step through almost everything and can save you the cost of it the very first time you repair your machine yourself versus letting the dealer have a crack at it
A few others things will make any job go a little bit faster and should help prevent some issues....
1) Get the correct manual for your machine
2) Get the tools you need BEFORE you start
3) Make sure you have all the parts you need BEFORE you start if possible. With a piston needing replaced, until you open the engine up you wont really know what will need to be replaced, however have as many of tehe parts you need on hand for most jobs.
4) A clean work area and hopefully a clean engine/part to work on.
There are a few tools for this job that you may not have and should either buy or rent and have a working knowledge of what they do and why they are used.
Beyond the typical sockets and wrenches, you will need: