I'll take a shot at this.
What to bring for signatures. Well, I've been to a good many conventions and 'the stars' seem to like to autograph pictures. That's not to say an autograph book is frowned on, but if you've collected a batch of 8x10 pictures with autographs, (IMO) it looks nicer and goes better in a binder :)
Oh - a hint from an athlete I heard. Ask them to personalize it - as in "To Dave" or "To the Smith Family". Some celebrities prefer that because it means the autograph will NOT show up on eBay. Regrettably, there are hounds out there who get kids to stand in line and get 'generic' versions of autographs so they can re-sell them. Some celebs don't care, some do.
Photo ops. If you paid for an autograph and a professional/stock photo, I've yet to meet one celebrity who WON'T take a moment for a quick picture from a digital camera. Again, some celebrities don't care, but show respect. I have a picture of Jason Carter (Marcus from B-5) picking up both my daughters, one in each arm, when they were 10 and 5 (now 23 and 18). He was one of the most personable stars I ever met.
A Star Trek uniform at a Star Trek convention? That's the only place where you can wear one and NOT stand out :)
Parties.. Depends on who runs them. I've seen some wild ones where they served "Pan Galactic Gargle Blasters" (stuff looked like Windex and tasted worse). I've seen others where they're just huddled around a TV watching a favorite episode. It's a crapshoot.
Remember one thing, though. You will meet a LOT of people and some will stand out more than others, and not always for good reasons. You'll notice some social misfits and some people with other social challenges. Don't let the minority ruin the experience.
Have a good time!