There are two main sorts of depression, the sort that comes from bad things happening outside of you and the sort that comes from inside you for no apparent reason.
The outside sort is when something horrible happens outside of yourself, which makes life very difficult to cope with. Perhaps you fail your exams, or your parents start getting divorced, or a friend commits suicide - dreadful things that just make you want to curl up inside and cry for ever. This kind of depression is painful, but medically speaking, it is also a normal reaction. If you didn't feel pain, you wouldn't be human. Eventually, the biting sadness wears off as you talk to people about your feelings, as nicer things happen, as life just goes on. It's still there, like a scar, but it doesn't hurt so much.
This kind of depression is when something collapses inside yourself. Gloom and greyness descend on your life for no obvious reason - everything might be fine, but you just can't see it that way. And if something slightly bad does happen, it feels like a crushing weight. This kind of depression is like an illness - like getting pneumonia or glandular fever. This depression is more difficult to cope with than the outside sort, and also takes longer to go away. For a few people, the depression is so severe that it controls every part of their life, and makes it seem as if it's not worth going on. If you can say to someone you trust, 'look, I don't know why, but I feel really, really down, and I need to talk to someone', then that is the first step away from this kind of depression.
If you think one of your friends is depressed, try to get them to talk to you about how they feel. You don't have to say anything or give advice, just listen. Don't be afraid of silence or tears. If your friend is feeling really bad, you perhaps ought to tell someone else about it - maybe your parents. Sometimes your friends may find it easier to talk to your parents rather than their own (unlikely as it sounds). If your parents aren't the right people, then try a teacher or someone else you trust.
Feeling sad all the time, of course. But sometimes people don't realise they're depressed and it comes out in other ways. How many of these fit you?
Having only one of these feelings, or feeling like this for just a day, probably isn't a serious problem. However, if you have several of these problems over days and weeks, then it may be depression. In that case, you should get help. If thoughts like these are making you feel that it's not worth going on, or that everyone would be happier without you, then you should get help right away.
Depression turns people inside themselves. It's very hard to talk to anyone about it, and that's part of being depressed. Talking to someone about how we feel, however, is very much the first step out of depression. Talking through all the horrible things going on in our lives is actually part of the mind's natural way of healing itself. The best person to talk to is the person YOU find it easiest to talk to. It might be your best friend, your mum or dad, your cat, teacher, doctor, priest or the lollipop lady - just someone you trust and that you feel will listen to you and understand.
If you don't have anyone close to you that you can discuss your problems with, then do please try your family doctor. If you go to the doctor, they will listen to you talk, and they won't mind if you just cry instead. They can arrange for you to see a counsellor, someone specialized in listening to people with depression, and helping you through it. They may get you to see another doctor called a psychiatrist who knows all about how the mind works and how to make it better when things go wrong. These people are excellent and helpful and can really make a difference to you. The doctor can also give you tablets to lift your mood - but only if the doctor thinks that they will help. Whatever you say to the doctor will stay completely private. They will only tell someone else about it if you want them to, or if they think you're seriously close to harming yourself.
Oh no, I've got another spot. But actually, you can't really see it, and I've got a tube of cream to zap it. And spots haven't stopped Kate coming to see me, which means maybe she likes who I am not what I look like...
God, my parents are awful. But I can't be as crap as they say I am, because when we did The Catcher in the Rye I wrote a really good piece for GCSE English about how much I hate them. Actually, maybe I can help Sharon face her mum's awful new boyfriend - I know what she's going through.
I'm so ugly no one will ever fancy me. But my friends seem to keep coming round, so I can't be a horrible person. Actually, judging by the birthday presents I got, they quite like me. And I might not be as pretty as Emma but she's a right snotty cow and I'm not. Well, not all of the time.