There are lots of factors you have to take care when racing, as far as i belive driving through the ideal line is the most important one, then braking points and turn in points, taking care of your tires is important too, you can be very fast for two laps but making your tires overheat.
One thing I do when I want to improve in a track is to use R3 tyres and 100% of fuel, then drive for some laps trying to find the way to go out of the corners the fastest I can, so when I use less fuel I can handle the car much more easier.
A common mistake is to turn in too early, you could try to brake about a third of a second earlier that you usually do, then brake in a straight line parallel to the outside side of the track, release brakes and turn in when you feel that the car can make the sharpest turn and change direction to the inside of the track, as you reach the apex press the gas pedal. This isn't the best way to take the corners, but if you don't master your braking skills it's the easiest way to learn, take your time to do every thing separated the first time, if you try to do everything at once you'll surely fail.