Throttle back your engine to get down into the green range of your RPM indicator. As you level out, you are transferring more of your thrust into speed instead of climbing, which makes you go faster. As your speed increases, the airflow over the wings creates more lift. The added lift, in turn, makes your nose rise up and your altitude increase. In order to maintain 2,000 feet, you must apply more and more forward pressure on the yoke to counteract your increased lift.
Trim tabs are like very small control surfaces on the trailing edges of the elevator and rudder.
They can be locked into a precise position to create a constant input that mimics what would happen if the controls were constantly applied. By setting the trim tabs properly, you can release the yoke and the plane will fly relatively straight and level. To set the trim tabs, first get the nose level with the horizon by applying forward pressure on the yoke. As you increase the trim, you'll notice that the yoke requires less and less pressure to maintain level flight. When you can let up on the yoke and the nose stays level, you have trimmed out to your current speed.