1. You need a tool kit it has to be in your inventory. There's usually a Tool kit on your car inventory, if not you can buy Tool kits from Fuel stations or from general stores.
2. To use Tool kit you need to stand next to your car and press Windows key which then says if you are carrying a tool kit to "Repair Car"
3. I guess this sums it up if you have any further questions feel free to message me via Private Message or comment on your thread!