Diet will basically be the root cause of this. Look at your caffeine intake (coffee/soda) and the times you eat and specifically what kinds of things you're eating. Crap processed food and fast food is going to make you tired. Fresh food and fruit/vegetables are going to give you decent energy throughout the day. Make sure you have a big breakfast too and drink a lot of water throughout the day.
Don't rely on energy drinks or any of that crap to get you through the day - that's a bad habit to form.