Well, I'm depressed for years now, but nobody has really noticed.
I tend to hide my feelings behind a fake smile when my family's around because I know that if they find out about me harming myself, having suicidal thoughts etc, they will lock me up in a hospital.
And I'm not going to tell them. No way. It's their fault for what I've become and the only thing I want from them is to stay out of my life.

So, I don't think it's a good idea to tell your parents. Your mom will get so stressed and you will probably feel worse after that. If you want to talk to someone, pick a friend who will take it seriously and will advice you, but, don't forget, we are here for you too

I just told my parents "I need to talk to someone, but I don't want to talk to you guys about it because I don't feel comfortable with it. Can you make an appointment for me to talk to a therapist, just to get some stuff out?"
You don't have to say you're depressed, but just say you want to talk to someone.

If your mom takes it that way, just reassure her how much she did for you. Make a list of things in your mind to have ready of good things she's done for you, how it's benefited you and let her know you really appreciate it. And also tell her that there are some things out of a mother's control, and that it's not her fault. If you end up talking to a therapist, they'll be able to tell her the same thing.

Telling my parents and getting the help I needed was a great choice for me, and I'm glad I went about it the way I did. I didn't say "I think I'm depressed" but I downplayed it a little bit, talked to someone, and then they explained what was happening with me to my parents, which was a lot easier than me telling them.
If you're depressed, it's important for you to get help. It can get worse if you don't deal with it. I know you're worried about your mother's reaction and how she'll take it, but think about yourself. You need to take care of yourself, and if that means your mom feels guilty for a little while before she begins to understand it's not her fault, then so be it. Do what you need to do to help yourself.

I told my mom after I got out of my depression, or, when I felt better. I never felt like I wanted to tell her when I was actually depressed because I just felt.. Different, I felt like it was abnormal and obviously thats the last thing someone wants to feel.

As for telling your parents, I'm going to have to disagree with sugarcube and say that you should tell your parents. When you tell your mom, let her know that no matter what she would of done, you would of still had depression. Its a chemical imbalance that you can't control, that no one can control. Just reassure her that she didn't do anything that would of caused your depression, its just something that happened, and its something that can get better and she can help you get better with it. Tell her that you've been thinking about going to a therapist, and let her know that you'd really appreciate it if she could take you.

As you said yourself, there are many advantages to telling her, and you shouldn't change your mind about that because it would be a disadvantage to yourself.

Make a doctors appointment mate. Your mum doesn't have to go and the doctor can't tell her. Remember suicide is a selfish act so please don't do that. FOr any one who is depressed I say go out to a different country do some volunteering. Did it when I was 18 and stayed in Thailand lol and got my self a paid teaching job. Been here two years now.