The most important thing is to decide what character you're aiming for and the amount of money you want to spend. Costuming can be expensive and time consuming hobby.
Make a list of your potentials and decide whether:
Can it be entirely bought or assembled from garments you have/can purchase?
How accurate do you want to be? More accuracy = more expense and more time
What skills do you have? Can you sew or are you willing to do some sewing? (Or do you have someone who will do it for you?) Or perhaps you're a whiz with the hot glue gun and intend to glue everything together? There are some amazing costumes made this way.
Do you want to stomp around in some armour for the event? Armour (made from cardboard to craft foam) is great but I think it tends to be the more complex portion of costuming and it's not something I've attempted...yet. If you're feeling brave go for it.
Also visit the
http://www.therpf.com/replica-costumes/.
There are some very helpful tutorials and posts from people as they show their progress from amateurs to near-on professional builds.