If you haven't, you should make a doctor's appointment for a check-up and tell them you're always tired and have no energy and would like to rule-out a medical reason (thyroid, anemia, hormone imbalance, etc.) *Ask about being tested for sleep apnea!!*
Start there. If all of your labwork and tests are normal, consider other causes: staying up too late, an irregular sleep cycle, poor diet (i.e. a lot of sugar, too little protein, etc.), depression (even mild depression can make you tired)...
I am tired all the time, too, but I know the reason why. (For me, it's a combo of staying up too late because I work 2nd shift and don't want to go to sleep for hours after I come home and also depression, but as long as I take my antidepressant EVERY day, I feel much better/less tired....)
Good luck in your quest for answers...