You stole my moms rental car and crashed it what should i do because im only 18.?

well seems to me that you wasnt really thinking at that time was you? if it was me i would tell my mother because of many different reasons

1. she is gonna find out anyway

2. why lie when you have better things to do in your life then hide it from your parents

3. they will never trust you again if you hide it

4. tell her and get it done and over with

but i warn you if your mom seems a little pissed off id give her time to cool off lol.