I have to do something - я должен сделать что-либо .
Нужно , надо, необходимо - those word express necessity , that exists .it could be for some person , Мне надо , тебе надо . It could be impersonal - Надо сохранить природу - the nature has to be saved . It could express something that person needs in , Ей надо помочь - she needs help , it could be what some person has to do - Мне надо идти на работу - I has to go to work