How do you keep your hair untangled AND not greasy!?

My hair is always tangled after like 12 oclock at school!i have tried conditioner but now my hair is greesy! i stopped using it but then my hair gets tangled again! i blow dry it every night too! idk wut the problem is! please help i am desperate! also if you have hair ideas when wearing it down please leave a tip or link! IM DESPERATE!