I do not have much energy
Are you having trouble carrying out normal daily routines?  Could be chronic fatigue syndrome.
I am thinking so, maybe i need to see a doctor on this
It isn't something that can be cured, but chronic fatigue syndrome can be managed.
How can i manage it
Either use cognitive behavioral therapy, or start exercising more.  Or both.
i will try and do both. But what is the cause of this
Nobody knows yet, man.  I'm sorry.  Some scientists think it's biological or genetic, and some think it's psychologically based.  
I think mine is psychologically based.
A doctor would diagnose you based on your symptoms.  You said you were tired.  What else is wrong?