haha, thats really cool, I'm 14 as well and I've just been to my aunts wedding a couple months ago, I guess you live in the US so you could try these stores:
Steve & Barry's- They have two really good lines
1) Bitten by Sarah Jessica Parker- It has some really cute summery dress which would be perfect and they are all under $20. The website is: www.bittensjp.com. I LOOOVEEE the clothes in this line :)
2) Dear by Amanda Bynes- I haven't looked there much, but I'm sure you'll find something you like everything is under $20 as well. www.dearbyamanda.com
You could also try Forever 21 (www.forever21.com) or H&M (www.hm.com) if your looking for something stylish and cheap. I suggest you don't buy too much of an expensive dress as you will not wear it much. I got mine from H&M
But if you are looking for something more expensive you could try Nordstrom (they sometimes have big sales) or Club Monaco (bought a dress there last year), its not exactly not expensive, but the stuff is really good quailty and it doesn't go out of style. Just be careful though at Club Monaco cause they usually focus on black and white, and you don't want to out stage the bride by wearing white. well since you have 500 dollars to spend you could try looking at Vera Wang, Erin Fetherson (sp?) who makes adorable dresses, Marc Jacobs or something designer.
Also you could check out some vintages shops if there are any near by if you want something unique.
Hope that helped & good luck finding the perfect dress! :D