Disco themed party?
I plan pretty much all my mates' parties. I get such a kick out of it, and they're always really fun!
Since you said you want disco, my inspiration for your party is Saturday Night Fever (classic movie!). You can get most of the stuff at dollar stores, for the floor decoration you can get it at a hardware or art store.
For added fun, make it a fancy dress party (a costume party) with the 1970s as your theme.
-get loads of gold and silver streamers, hang them so that the entire ceiling and walls are dripping/covered in them (not hanging straight down, but making arc-like shapes) - do the door ways so that they're hanging down like beaded curtains
-make a dance floor: if you have wood or tiled flooring, get a big square piece of cloth (like an old white king sized sheet) or a tarp that paint sticks to (big enough to dance on) and paint it in bright various-coloured square pattern (ie red square, blue square, purple square) then firmly attach it to the ground (your parents can help you figure that out -- I can't, since I don't know the layout of the party room). Make sure that it's tight so that it won't come up when people are on it!
-get *lots* of those metallic silver and gold balloons from the dollar store and rent a helium pump - fairly self explanatory: pump them up with helium; fill about a quarter of the balloons with regular air (that is, if you buy 400, do 300 with helium and 100 this way). The non helium ones will mill about on the ground for a nice effect
Food: Regular party food will do, plus you can't forget your cake! I.e. silver and gold coloured bowls of crisps and nachos, trays of fruit and veggies with low-fat dips (healthy foods like that were a rising trend in the 70s), various sodas (remember to get some diet as some people dislike regular!) with ice and clear disposable cups (I usually have permanent markers by the cups so that people can draw pics or their name on their glass, so they don't lose track of it)
You might also want to get sparkling apple juice for cake time, so it's like you're all having champagne
Music: the most important part!
-Ask your parents, aunts, uncles, teachers, and so on about their favourite disco music from when they were kids.
-Your parents might even have some old disco records you can borrow. If no music is on hand, drag your parents or another older relative about to discount bins and pick up some old tapes or CDs of disco music.
-Listen to the music with a friend or two (whoever's helping you with your party) and make a list of all the songs you'd like to have play at your party, whatever you like or think the others would like
-Have some modern music your friends all like on hand as well in case they don't like the disco music!
-Have somebody (ie a cousin, older friend, even a parent) play DJ for the night and their job would be to play the music, take some requests if you'd like and the music's on hand. If that's not possible, make a playlist and have it on (ie plug in your ipod to your stereo, or burn some CDs).
Gift bags: If you want to do gift bags, keep them fun and cheap. Go to the dollar store and get some noise makers, temporary tattoos, silly stickers, a pack of gum, some cheap pre-packaged dollar store candies, some fake money and so on, and put them in cheap party bags.
Hope you have lots of fun and happy birthday!!!