Hey there, I know first hand depression is hard, but are you really sure you are getting it again? A headache and loss of appetite could be merely due to an illness making you feel really run down causing you to let your guard down and feel weak. It happens sometimes. However if you are depressed, then I suggest the first thing you do is tell someone, don't wait or hold back for it to get worse, if you don't tell someone you will be stuck going back and forth with depression.
To get depression sorted out, you need to actually think back, as far as you can remember, just take an hour or two out of the day, relax, meditate, do what ever you need to do to have enough time to think back to any time where around you feel you started to become unhappy. Another way to find out what is really going on is therapy, there are all kinds of therapy from listening therapy to hypnotherapy, we're all different so we need different things.
I also recommend talking to one of your parents about how you're feeling right now, as bad as it may sound it might be the easiest way to get started on figuring out why, what caused this, and how to stop this from going any further or happening again because depression affects hundreds of people each year who don't get it sorted and they live in self pity. I know you're stronger than that.
Thinking positive thoughts is always a start, a good thing to have is a goal to achieve or something in the near future you're looking forward too, this will give you something to keep your head up high for. If therapy is out of the question, even your doctor may just be able to help, I wouldn't recommend starting anti-depressants as there are ways in life to deal with problems without prescription drugs to try to just cover the problem up.
! Have a goal
! Look forward to something
! Find a new hobby
! Keep that smile on your face
Good luck!
-Ally.