Your vehicle isn't the only vessel that needs repair! You will find yourself being injured here and there; Whether you got hit by a car, fell off a roof, or got shot by the local gang members.
Getting healed will cost money.
Visit a hospital - Talking with the hospital NPC will give you the option of healing to full and also buying consumable items (which will be located in your inventory under 'gear') you can use on the go! Like health packs, for example.