I agree with shagino. Step 1 is always to go to the local track. Find out what class has the most people. Find out what brands all those people run and what the local hobby shop stocks. Buy that.
You want to be able to race with a large group. It's just more fun. Plus, as you get better, you'll move up in the standings and that feels really good. If you buy the brand everyone else has, everyone there will know how to setup your car. That helps a ton. Being able to get good advice and setup help will make the sport a lot more fun. When you break on race day (and you will eventually) it helps to be able to buy a part on the spot so you can keep racing. It's terrible when you break and you're out for the day because you bought some brand that no one stocks and you have to buy parts online. Also, tires are the most important part of any setup. Ask around the track and see what hooks up best. Buy those tires. You'll have so much of an easier time getting good at racing if you have the right tires.
So go to the track. See what you like. Watch for a whole night. Volunteer to turn marshall. By the end of the night you'll have a good feel.
As far as building a kit vs RTR.... I'm in favor of the kit. You'll know a lot more about how to tune and fix your car if you already built it once. If you can easily build ikea furniture, you can build an RC car. You'll spend a bit more this way though. An RTR will get you running right away and you can always upgrade it as you go.
If you just want to bash in the street and have fun in the backyard, you can't go wrong with a Traxxas Slash.