Theres going to be alot of freaks, and alot of anime fans gathered together taking photos. It has a totally different vibe from large comic conventions, as these people have pretty much created their own sub-culture to indulge themselves in.
Wear whatever you want, but for best experience wear any shitty costume based off anything relating to animation, video games, or technology. You're going to have to be familiar with terms such as "groping" "free hugs" and "Haruhi Suzumiya dance".
Other than that, if you lay low you should be able to survive.
I've been to alot of conventions, as far as I can remember in order:
Anime Expo 2002, DigitalLife, NY Comic-Con, Wizard World Philadelphia, Big Apple Con, Super Mega Show, DigitalLife, Big Apple Con, NY Comic Con, Big Apple Con, Chiller Theatre, Wizard World Philadelphia, San Diego Comic-Con, DigitalLife, Big Apple Con, Monster-Mania
Recently, I've started going to these places mainly to build up my autograph collections, in conventions specializing in guests and stuff however, I usually have to pay. However in the bigger cons and in booth signings, they are usually completely free. San Diego Comic-Con was a blast, however it was one of the most frustrating, most tiring, most busy ass cons ever. For 5 days I'm going around checking schedules, hogging more bags and poster tubes then I can carry, trying to use my time efficiently and scouring the HUGE conventions floor trying to figure out which signings are taking place at where. Unfortunately I missed a shitload, as I cannot be in 5 places at one time.
I prefer smaller cons, however they always lack the guests and freebies that bigger ones have. The larger ones usually frustrate me more unfortunately.