You don't really need decorations, but if you want to make it seem more like a real party, feel free to put some up. You don't need a ton of guests, if at all. Just invite a few of your close friends over. Make sure to have a really yummy thing for the cat, instead of a birthday cake. Perhaps a can of wetfood, topped with treats. You could have a few cat-themed games for the guests, such as: cat charades, name that cat, etc. etc. You should also have some games or toys for the cat. Put paper bags on the floor, tie paper wads on to door knobs, put catnip on the scratchers, there's a lot of things you could do to make it fun for your cat. If you are giving your cat presents, make sure you give them toys, treats, or sleeping things. Also, don't pay too much attention to your cat. It may not like too many people. Well, those were some ideas for a cat birthday party!