Good thing, you spoke out in this forum and there show up many other people like you. It is not bad, we get depression. First thing is, accept it - it is just a disease. Second step - you should learn about it. Read about it, get more information and seek treatment.. 

In order to fight depression, one needs to go out, do exercise, but it is soo bad, one gets just tired and prefers to sleep all day in dark.. that worsens the condition and so on.. it is kind of vicious cycle.

Don't think it is just emotional thing. In fact, what we feel is a effect of what is going inside our brain. Normally people with depression has lesser amount of active neuro-transmitters in brain. Taking vitamin B (B6, B12), Omega3, 6 oil (fatty acids) helps. In order to get motivational, listen some therapeutical audio. (one such is Lucinda Bassets "Fighting anxiety and depression" audio series (you can download from torrents :).. 

Let me give you some tips to fight this -

1. Take quality multivitamins, (B complex), with vitamin D and Calciums. Deficiency of Vitamin B complex, folic acid, iron, potassium or any of them causes depression. Start taking quality multivitamins and minerals. 

2. Fatty acids (omega3 and 6) are MUST for brain. In fact 70% of our brain is made of fatty acide. Get quality fish oil capsuls. It is good antioxidant - burns your fats and keeps skin healthy.

3. Exercise - either play some game, swimming, or whatever.. Take plenty of drinking water.

4. Sleep - sleep is medicine. Sleep in time and sleep enough. You will feel the difference- you start thinking clearly, feel more energetic. Never sit infront of computer or tv after ten. Sleep deprivation is the major cause of daytime fatigue and tiredness. 

5. Trust on some good friends. Make few friends close to you. It might be difficult initially - but some people are really understanding and they help you. 

6. Listen inspirational speech, stories, movies. 

7. Avoid alcohol and smoking - it kills brain cells.

8. Avoid sweets, caffeine, fat foods, soda - they harm your brain. 

If you act with discipline, you will start feeling the difference in one month. You feel more energetic and want to learn stuffs. It is a constant fighting - takes some time. But you can have quality life back.  

There are some good herbal medicines too, but their effect has not be standardized. One of them is SAM-e (google it). It is good mood establisher. Not sure where do you live. It can be bought over the counter in USA and is a prescription drug in Europe. You might wanna try it..

NEVER USE ANTI-DEPRESSANT DRUGS. They might have reverse effect. Try it as a last option. 

Final note - what you are feeling is not usual. You can care it. You can overcome it. And you will have your quality life again. Dont give up.

Best of luck!