External Flashes/Flashguns?

I am getting more into lighting with my photography.. And I am looking into external flashes/flash guns.. but I don't COMPLETELY understand them. What do I need to set two flashes and have them fire when the camera fires? Do I need a certain type of flash? I use a Canon T1i and haven't worked with lights (flash, strobe, ect.) I shoot a lot of bmx/skaters and lots of portraits.. So if anyone has any tips for buying and using external flashes with a wireless unit to fire them, it would be greatly appreciated!!