OK, now you people are starting to annoy me... some people learn more slowly, and just imagine spending 16 years only in orbit and suddenly finding yourself in the atmosphere, well this guy is in the same situation, it's hard to forget such things after so much experience. I agree that we have done a lot for him, but not everything. And just telling him what he needs to do won't work, you need to explain him first why and only then tell him what to do.
And now, back on topic. Rocketdued, you are going too fast. There is no way to slow down in space like in the atmosphere, you need to burn your engines "the wrong way", that is, backwards. Backwards in orbit is called retrograde. And, in space, you also need to time your burns. You are going too fast when you are at your periapsis, the lowest point of your orbit. So that's when you need to slow down.
So what can you do?
First, check your PeA, the altitude of the lowest point of your orbit. That needs to be high enough, so you don't hit the air. If it's under 150 km, then it's not good. You'll need to raise your periapsis. Wait until you are at your apoapsis, the highest point of your orbit, and press the big Prograde button, which will point you forward. Then, just burn your engines, but not too much, or you'll overshoot. Just burn until your PeA is somewhere above 200 km (if it's more, don't worry, as long as it's not very high, it's OK) and then turn off your engines. Then, wait until you reach the periapsis, the lowest point of your orbit, and turn retrograde. Now it's time to slow down. Make sure you are pointing backwards, and when you are close to your periapsis, engage the engines! And then burn until your orbit, the green line in the Orbit MFD, is round.
Tip 1: You can press and hold Ctrl - + to make the engine level slowly increase. It's good to let them burn just a little, so you have more time to think.
Tip 2: By waiting I mean accelerating time with the R and T key, of course... Just don't overshoot.