Generally you can just enter whatever catagories you want (although a specific contest my have exceptions to that) just to be considered for the overall Grand Champion, you have to enter all. Most charge you $ per catagory entered. I am cooking in an FBA contest this Friday/Saturday and the entry was $50 per catagory.........so it will cost you $200 to have a shot at the GC...........Then there are folks like me who enter all the catagories, pay the money, and still don't have shot, but that is a different story. LOL
Doing one catagory will give you a feel for how things work as well as for how competive your product is.
You can enter by yourself, just pay your money and get your entry done and turned in on time.
Like Texas-Hunter said, practice at home and make sure you can get your stuff done on time........including making the presentation in the box, cooking is just part of it.
Good luck.