Most Beginner levels are quite simple to get under par time on, and you'll get under par by just doing a clean run without any falling and just keeping your pace up (some you'll get it twice as fast as par that way).
The only level I had to try a few times for myself was level 19. What you do there is as follows:
Go left, turn the camera right and head onto the ramp. Jump at the top and gather the gem on the block.
Move on a little and make a u-turn to the left onto the next ramp. Don't jump this one or you'll go over the gem.
Another u-turn to the right takes you to the final ramp. Jump it to a bridge. Then get to a standstill on the corner, jump to get the gem, and head for the finish.
If you do all this fast enough, you'll get under par time.