Not enough exercise can lead to a lack of stamina therefore you get tired easily. As tat says, not enough food, or too much of the wrong foods - you need a balanced diet, not too many fats or carbs. Too little iron in your diet can leads to anaemia which also can produce tiredness. As well as physical illness, your mental state can make you feel tired - particularly depression, or stress which can drain you of energy.
If your tiredness has been going on for a while, like a few month, and you are in the second half of your life, it may be worthwhile to have a bloodtest. You could have pernicious (megaloblastic) anemia. Nothing to worry about, but you'll need a periodic B12 jab.