Derry,
How you use your brakes is largely part of your driving style. Some racers program what is called "drag brake" into their cars which causes them to kick in when you release the trigger to neutral position. I don't think too many people use a very aggressive drag brake though. A slight amount can be helpful in jumping but too much can make your car nose dive in the air and act erratically while cornering.
As far as when you should actually apply the brakes, most of your braking should be done while your car is traveling in a straight line. You should brake at the end of the straight until you reach the appropriate corning speed, release the brake, and then apply throttle to carry a constant speed through the corner. You should accelerate after you apex and start exiting the corner. While you are in the corner, you can use the brake to help your car rotate. As your skills increase, you will be able to brake later and drive into the corner while braking. This is called trail braking but is a bit harder than braking in a straight line and may cause your car's rear end to spin out as the car may oversteer.
You should just experiment and see what works well for you. Braking later is harder but is a faster way to get around the track. Braking early and straight is easier, not as fast, but will get you around the track without stuffing your car into the pipe.