Tap the map, tap "Services", then tap "Weapons". Once there, you can purchase more health, or the pistol, SMG and shotguns. This is useful while surrounded by the police and are running low on ammo. You can also buy cars, but they are mostly at parking locations.
-During the start of a race, tap your opponents to lock them in as targets, then shoot them until they explode. You can now take your time in the race and win every time.
-Before a race, get a gun that is capable of a drive-by shooting. Go to the starting line. When the race starts, allow your competitors to get in front of you. Stay close to their cars and repeatedly tap to shoot their vehicle. The car will eventually begin to burn and explode.