honestly, i would say do NOT move in with a girl that you dont have many many manyyyy years of acquaitance and romance with... I cannot tell you how many of my friends dove in to move in with someone only to then be eating out of trash bins cause their relationships went south and they are stuck on a multi year lease / mortage on a home they can no longer afford.
Odds are greatly against you. I'd think looong and hard about making a leap to move out of your parents house into a scenario like the one you described.
Youre fresh out of high school, stay at your folks' place. Get the ball rolling on your career or college and get that on a steady pace. It's stressful enough to be finished with high school and reach that major crossroad in your life. I'm sure your parents arent shooing you out, and if youre not totally miserable at yourmom's then just stick around for one or two years to get your wings.