Look, GT is a game not just about trading and racing, but about setup. If you buy for example Turbos, the hp dramatically rises thus the car goes faster. Tyres on the other hand give more grip. I'm not going to go through all of the different components, but buy sort out the handling first (unless you are a racing supreme). Check out others laps to compare if you want (Challenges & Races). Everything has it's purpose, and if you read the info by the parts, you can find this.
After that, when in the race pre-start menu, go to settings, and you can adjust the settings of the car (ride-height, camber etc.) This isn't as important at this stage, but you can also take off/put on parts which you have bought. From the fact that you feel they cost a lot suggests to me that you don't have much money (I have 99mil.-sry to boast). You will find that these changes make it easier to win (and get more money). Then you can buy more parts, making the car better, and win more races, and so on. That is the game of GT. Hopefully this will help you. If you have any problems, message me.