My personal advice list for cosplaying in general. This is mostly for new cosplayers who don't have a lot of experience, so if you already know how to sew and have a lot of free time, then just ignore me.
1) Keep it within your budget: I would say set how much you want to spend on the costume and try not to go over it. Thrift stores and online shopping would help a lot with this. But expect to spend somewhere in the triple digits, unless you are just awesome. (And if you can do it cheaper, then please post some hints for me!)
2) Keep it simple: This comes across in a lot of ways. If this is your first costume, then you might not want to choose your all-time favorite character, and instead just choose someone with a relatively easy and recognizable costume.
3) Don't get obsessed with accuracy: (I've fallen into that trap before) I guess this is part of keeping it simple. Don't freak out about making it look "exactly" like the character.
4) Make it easy to wear: This is my personal rule, because I sweat a lot. Remember you will be walking around a convention all day, so at least consider giving up the six inch heels or otherwise uncomfortable shoes, and also consider how much you might perspire in the costume. If this doesn't matter to you, then that's fine. Personally I had an elaborate costume for my first DragonCon that I didn't wear because it was too damn hot (September in Atlanta and I had to walk outside to get there.)
5) Don't listen to haters: Some assholes might say your costume sucks no matter what you do. They are assholes and deserve exactly none of your time and attention.
6) Don't feel obligated to listen to anything I just said. :-)
7) Have fun, and when you finish I hope you post pics up here on Iwaku!