If you look at the sectors (boxes divided by grid lines on the map), when there are planes above 500 feet in that sector, they will show up on radar as dots. If you have your icons set on default, green is friendly, red is enemy. They also show up in the upper left corner of the sector they are in in the form of a bar, like on a bar graph. The bigger the bar, the more planes.
If you are really new, I STRONGLY suggest you spend some time in the Training Arena where you can learn the important stuff a lot easier and quicker. I realize it might not be as much fun as soon as going to the main arena, but it will make going to the main arena more fun quicker when you are ready. You are MUCH more likely to find some one who will be willing to help you and answer your questions without being a jerk about it. People in the Training Arena will have more time, be more helpful, and be more patient.