Many beginners to Desktop Tower Defense are confused by the scoring. Here's how scoring works for the normal mode:
For every creep you kill you get a certain amount of points. The number of points you get will be displayed in red when the creep dies. If you kill all the creeps you get 4992 points.
There is a bonus of 25 points for each life that you have left at the end of round 50. So if you kill all the creeps, you get a 500 point bonus for this.
There is also a bonus for sending in the next level of creeps early. For every second left on the clock when you hit the "Send Next Level" button, you get 2 points.
So you can get up to 5492 points for just killing the creeps, the rest of the score needs to come from the time bonus. The top players are getting over 8000 points. Good luck!