How do I know if I'm Pregnant?

I am 14. About a week ago I got drunk with two of my friends. Nobody else was around. I'm suppose to be on my period but I'm not. My friends said I also smoked pot. I have been eating a lot. I was wondering if the eating and non-period is due to the alcohol and pot? My friends said I didn't have sex, and I am still a virgin, but maybe my friends are lying. I wont ever drink again, that was a first time, I am also against drugs. I just needs to know how soon the signs come in and what the signs are. Thank you =]