Hi and welcome!
As Gail wrote, flashes are complicated. There are many operation modes, going from manual, to automatic, to through-the-lens metering (TTL), with or without power adjustments, etc. There are flashes with faster or slower recycle times, some with longer reach or witha swivel head. There are flashes made by Canon (your camera's manufacturer), others made by Sigma (a well-regarded third-party manufacturer).
If you want to this right, the best solution would be to get a flash as advancedand features-rich as you can afford, but even then the choices are not simple.
I would suggest visiting the Canon DSLR forums at
www.dpreview.com and maybe also visiting the Lighting forum.
Lastly, I mention that flashes and weddings don't always go well together. Quite often, the minister will lot accept any flash firing during the ceremony. Weddings are a very complicated (and very emotional) event to photograph, and you get no second chance. If you do plan on using a flash, make sure you understand how it works before the big day. An external flash is not a plug and play device, sadly. The best thing is to learn from someone who has already done it.
