Experimenting's the easy part. Analyzing and understanding the results is a little more difficult if you don't understand the theory behind it.
Welcome to GTPlanet, sangalang. If I can guess, I bet you've added all the power upgrades but youre still running on stock tires, correct? If so, take all the power off of it and add T2 or T4 racing tires. The rubber is the first place to start, because it's like upgrading the acceleration, braking, and steering at the same time.
If you're new to Gran Turismo, here's my other word of advice: brake earlier. I never felt in control of the car until I began to brake earlier than I had been by far.