The lens that you need is the Canon EF24-70 F2.8 L USM. The bad news for your friends is that this lens costs $1100. You also need a Canon 550EX Speedlight and a Canon CP-E2 battery pack to power that flash. This combination of equipment will all attach to your Rebel and will make it possible for you to do an adequate job at your friends wedding. If you try to use the ambient light inside the church you will have to shoot from a tripod and that will prevent you from being able to maneuver to capture moments like the first kiss. Wedding photographers make a fortune because it's the most difficult and stressful type of shooting that there is.