Get an external flash unit where you can tilt and swivel the flash head. You'll need this so that you can bounce the flash off the ceiling or walls.
If you can afford it, get the Canon 580EX II flash because it has more power and it can be used as a master to control other Canon flashes. However, if you're on a tight budget, get the Canon 430EX II flash. The flash head meets the minimum requirements of being able to tilt and swivel and it's also lighter than the Canon 580EX II flash.
As far as accessories go, you should get the Canon STE2 Speedlite Transmitter especially if you're going to shoot with only one external flash unit. The Canon speedlite transmitter enables you to shoot with the external flash unit off the camera. Try it out and you'll notice a big difference in your pictures!
The other accessory I highly recommend would be the Sto-Fen Omnibounce. It's small, lightweight, and very affordable. I've tried the Gary Fong diffusers and I found them too big and bulky and the quality of light wasn't all that much better than a Sto-Fen Omnibounce.
I actually use the Canon Speedlite 430EX II, Sto-Fen Omnibounce, and a Canon STE2 Speedlite Transmitter when I'm doing nightclub photography gigs. I only use the Canon 430EX II even though I also own a Canon 580EX II because of the weight. It can get very tiring holding the camera with my right hand and holding the flash unit in my left hand. The lighter my overall gear is, the better.
Learn more about flash photography from the following sources below.
- On-Camera Flash Techniques
With this guidebook, photographers learn how to create stunning, professional images while avoiding the common pitfalls of using an on-camera flash. Techniques for using simple accessories—such as bounce cards and diffusers as well as how to improve a lighting scenario by enhancing it rather than overwhelming it.
- Scott Kelby's Digital Photography Boxed Set
Scott tackles the most important side of digital photography--how to take pro-quality shots using the same tricks today's top digital pros use (and it's easier than you'd think). Each chapter is packed with plain English tips on using flash, shooting close up photography, travel photography, shooting people, and even how to build a studio from scratch, where he demystifies the process so anyone can start taking pro-quality portraits today!