ok, calm down lil man. while driving around, you may notice colored streetlights(red, blue, green, purple, etc.) those are hints that a shop is near by. when you find a shop, you can by upgrades.
yellow-car specialties shop
green-body shop
blue-performance shop
light blue-car lot
purple-garage
you can also just read the manual!
"One who is willing to give up a little freedom for a little security deserves neither!"-Benjamin Franklin