Beginner Singer Help Needed

I am in my school’s production of Little Shop of Horrors as Orin. I have done a play once before but this is my first musical, and I am not a singer at all. Could anyone give me advice or teach me how to read or hit notes or stuff? I have sheet music for my songs but it means literally nothing to me except the notes that are higher up means sing higher and the lower notes mean sing lower. My director also mentioned singing in my head voice which I literally have no idea how to do, she had me practice but my voice went really high and that’s not how I planned on singing because I sing more comfortably lower. I know I don’t sound good but I didn’t think I sounded TERRIBLE but I like to just burst out into song for fun and people say how awful I sound lol