I started drawing cars regularly at the age of 12. What you have is a good start. Keep working on your skills.
A good book for you would be
"How to Draw Cars the Hot Wheels Way"
I wish they had this book when I was your age! I think books are a great resource, as well as the internet. I sketch my cars, scan them and finish up in Photoshop. Keep it up and do not let
ANYONE discourage you!
Save all of your drawings so you can go back and gage your progress over time.
- DV