vehicle composition

[Rabd]

What material do you think I should make my car out of? last year I used carbon fiber but idk if it is the best thing to use.

[knightmoves]

"What material should I use" and "What shape should it be" are not independent questions.

What access to tools and equipment do you have?
What is your budget?
What expertise do you have?
What is your goal? (Are you trying to place first in a competitive state championship? Or are you just trying to place at a not-so-competitive regional?)

You understand that there's a speed advantage in having a lightweight vehicle. How much advantage? You can go away and compute the theoretical max speed assuming your 2kg mass falls 80-90cm. You also need to have reliable pointing / steering accuracy, and reliable and reproducible breaking. Does an ultra-lightweight car help you with those? That's something you have to decide.

If you did the event last year, and used carbon fiber (cf rods, perhaps?) to make your vehicle, how did that work out for you? If it worked well, and you were happy with it, there's no reason it wouldn't work well this year. If it didn't work out, what problems did you have? What can you do to avoid those problems this year?