If you always feel weak, tired and dizzy always then it can be because of multiple reasons which can be easily cured if you follow certain steps but the first should be identifying the cause of the weakness.
In most of the cases people who feel tired & sleepy are anemic which means your body is not making enough red blood cells which your body requires. The solution for anemia is changing your diet with iron, protein and vitamin rich foods such as fish, cherries, nuts and raisins.
Depression can also make you tired, dizzy and many people feel more sleepy when they are depressed. This is because when neurotransmitters are not balanced they affect your body and energy levels. The best treatment it getting out of depression and the way to do so may differ on the reason of depression. Usually meeting with good friends, going on vacations and doing what you love can treat the problem.