"I have a schoolbag on my back."
Translation:Eu am un ghiozdan în spate.
First, it would be "spatele" (spate + le definitive article). Second, it's obvious we are talking about your back, so you don't need to add the possessive "meu".
Examples where it is needed:
Am un ghiozdan în spatele tău (I have a schoolbag on your back - it's a weird sentence, but for the sake of the argument :D.
El este băiatul meu (He is my boy).
Ea este mama ta (She is your mother)
Acela este cățelul meu (That is my dog)
Ei sunt părinții mei (They are my parents), etc.
Lol, I know we are talking about my back, but I was just wondering if I could add "meu" without getting it wrong.