"I just want to tell you that i should leave"
저는 떠나야 합니다.
저는 떠나야 됩니다.
저는 떠나야 되요.
~ 합니다, 됩니다. I think you can use these after leave ( V) (more polite way )
~ 되요, : you can use this as well in general situations.
떠나야 ( should leave) + 됩니다.
떠나야 ( should leave) + 합니다.
떠나야(should leave ) + 되요.