I'm no wonderful expert or brilliant performer, but here's my advice.
Unless I'm wearing it as a partial, in which case I just wear normal clothes I personally use a cheap Underarmour knockoff I picked up from Wallmart a few years ago at Anthrocon. I'm usually sweating before I've even finished putting the full suit on, but the underclothes do a good job of holding the moisture in themselves and not letting the suit get wet and icky. It still gets a good spritzing with antibactieral "pet oder remover" after wearing it though
If you buy Underarmour, which is a brand a lot of fursuiters use, be careful when you buy it, there are two types and it can be hard to tell the difference.
One helps you keep cool, the other warms you up...
GET THE COOLING ONE! Having said that, I know some suiters who just wear a thin t-shirt and boxer shorts, not sure I would recommend that myself but works for them.
With the rest, my main recommendation is to take frequent breaks in the headless lounge and regularly sip water until you learn your own limit, everyone is different. The headless lounge is also a great place to find things out, I've got some great advice and tips in there just standing in front of a fan and chatting to people while cooling off. Fursuiters in general are a friendly bunch so don't be shy