Can someone help me with singing Glinda’s part from the wicked?

I was chosen for Glinda’s role, but the thing is, i CANT sing. I know the tone of the song. its just that when i try to sing, i have a bad habit of forgetting to breathe, and i tend to clutch my stomach when i sing high notes. Idk if this is a bad thing, but i also tried copying Glinda’s voice. No wait i do copy the tone of every music i listen to, to the point where it sounded the same in my head. I watched i few “how to sing this song” videos and those “how to sing opera or high notes” videos, and nothing really helped me. Probably because im doing it wrong. Actually when glinda sings high notes, i noticed that its kinda opera like? Idk. But in the scene “what is this feeling” her part sounded opera to me. The “popular” one has some high notes in it, SPECIALLY the LA LA part. I was like “no way i cant sing that” but i have no choice. I need some tips and help. I want to improve, i dont want to embarrsed myself infront of my classmates.