Preferably multiple times. If you go for an advanced robot then you'd better plan on spending at least 1 year working on it. That is probably the most important thing to remember. A well planned, slowly built robot will most always beat a hastily built robot. Besides you'll preserve your sanity that way.
Round bases are a better choice than square ones.
You should always put bumper switches on your robot to keep it from running into walls. You might also consider coming to a contest before your compete to get a feel for it. I thought long and hard about the best way to turn my existing base into a decent fire-fighting robot. Since the motors, wheel encoders, and processor were already constructed, all I had to put on it was a line detector, a sound detector, a candle detector and some way to follow the walls. To allow the robot to follow walls I constructed a rotating sensor beam that hung down from the top platfrom of the robot.