I do see a lot of young females who come to me and ask this question

My simple advice to you is

1. Drink water.
2. Drop all other drinks including caffeine and diet drinks
3. Walk 30 minutes everyday
4. Have a nice 6-8 hour sleep

You will find plenty of energy in a few weeks.

The busier I get, I drink more water. If you drink good amount of water you will not feel tired.

*I intend this information for a general population assuming no significant medical problems including anemia, thyroid abnormal, heart diseases, lung diseases etc. As mentioned in the disclaimer do consult with your doctor.