Last week I was in a motorcycle accident I wasn't hurt but do have a deep cut, how to stop scarring?

Scarring. will occur if the laceration cut through the dermis-the deep layer of skin. You can't stop scarring, it is a part of the process of healing. Do watch for infection, which can make any scar worse and take longer to heal.
Depends. Depends...That cut might of required stitches. If you didn't get the stitches and it is deep you may end up with a nasty scar. Generally, you want to keep wounds moist. Is the wound healed yet? Once skin closes you can apply something like mederma (over the counter scar prevention and treatment product.) there is also producs like silicone sheeting. (i am not sure off hand if they require an rx.).