This is a very good thread I started a while ago that has a ton of good information that you can read through. Barrel Racing Exercises and Drills.
There is a lot more to it than just running around three barrels, you're best bet would be to talk to a trainer or a coach. Possibly even take a few lessons just to get the feel of it and have that first hand experience before just jumping into it. It's also going to take a good bit of time and patience, don't let yourself become discouraged if it doesn't happen right away. Barrel racing is a sport that takes a good while to get into and become successful in.
Here is another thread that was started recently about barrel racing tack Must have barrel racing tack
A good bit of the tack is personal preference, as far as choosing saddle pads, bridles, breast collars, different types of support boots, bell boots.. but the absolute most important thing to factor in is that it all FITS. It doesn't matter if you have all of the most expensive stuff on the market..It'll do you no good if it doesn't fit properly..
If you're just wanting to do it for fun, you won't really HAVE to have a barrel saddle, even though it's recommended.. I always have a really nice saddle pad.. I use Classic Equine ESP pads.. Support boots, bell boots, a bit that my horse is comfortable in and responds to well.. ANY bit can be a barrel racing bit, you don't have to have all of the big "barrel racing bits"..