Depends what you want out of the game. The game gives you cars you can use initially, then, at the start of each race, there's a three car suggestion at the bottom right. Choose the third car. Keep your pp at least 100 points under the limit (keep it as stock as possible, changing all the non pp things first). Try to win.
I'm currently on my second run through of the game as GTP_Rossi and I'm trying to win with difficult cars. I used the motorsports elise for many IB races - fantastic fun, and now I'm using the MP4-12C maxed-out in the dream car champ on IA. It's difficult and great fun.
I'd like to use the yellow-bird but I have my limits. One day....one day.
Find a car that's challenging to drive. It will teach you something. When you've mastered that car, find a harder one and do the race again. You'll learn the track, and you'll learn to drive.
Oh, and never use racing softs unless it's raining