Ok so I'm going to the doctor in a couple of days, and I don't know what I should say...

I'd say be straight forward, but I've had some less than enjoyable visits with doctors, and I myself have gone to therapy and had the therapist mistake what I said and it caused some problems, but being depressed and having thoughts of suicide at and around age twenty to thirty is fairly common, and I've taken a class about this stuff.

I'd recomend just telling your doctor you've been feeling rather depressed lately, give a reason if you have one, if you don't let them know, from there hemay try to asses if you are a threat to yourself or others, which you can ask yourself now, have you actually attempted suicide, have you harmed yourself or others, are you a substance abuser. And just a bunch of questions like that, and don't worry so much, so long as they don't think you're a threat to yourself or others you're pretty much in the clear.

Your doctor may prescribe you something to help with the depression, suggest that you see a therapist, or something else along those lines. If your depression isn't terribly sever or frequent talking to someone about it may help, especially if there is/are any specific events that would cause you to feel this way. If you get prescribed something make sure someone (at least one person) that you see regularly and that knows you well, knows that you are taking something for the first time, sometimes anti-depressants can make depression and thoughts of suicide worse, in which case it may be a good idea to have someone near by to keep you from doing anything.

If you'd like to talk, or like me to try to offer you some more help and advice, don't hesitate to message me, I'm no therapist, but I know a decent amount, and may be able to help.

Actually you should be as straightforward as possible. Doctors are there to help you, not judge you or your reasoning. If you are feeling depressed, that is a valid medical condition.
If you don't want to just come out and say it, maybe just describe how you are feeling. (tired, anxious, stressed out all of the time, paranoid, thoughts of suicide, etc.) And he can make the medical termination.

There is no reason in not telling them everything. Laying it all on the table will insure you get the help you need and not the help you don't need. And don't worry, the doctor will have a system of questions he will ask you so you won't just be sitting there and have to blurt all your problems out once he walks in. good luck

You should be straight forward with your doctor. Mental health can degrade really quickly. I have been there, trust me.
Just start out with your depressed, the first thing he will ask you is, if you feel suicidal. Do you want to hurt, or kill yourself. The next would be if you want to hurt someone else.
Try to explain it clearly, even though I know your mind is a mess right now. Be honest, don't pussyfoot around the subject. If your suicidal say so, suicide isn't the answer, but it is easy to conclude that it is the only way out.

He might ask how your personal life is... How things are going with your parents, family, boy friend, etc. What you think makes you depressed, etc.

He will probably refer you to a psychologist for professional help, as a GP isn't educated enough to deal with suicidal people. Depressed people are lost, and the littlest things said can change their opinion. The psychologist might prescribe medicine(anti-depressants), and anything else you may need(anti-psychotics, mood stabilizers, something to help you sleep). Not saying your crazy, you just might have a chemical imbalance in your brain.
He won't conclude that you are a nut that has to be admitted to the psyche ward. As he isn't trained enough to conclude that. GP aren't majors in psychology.
And you are not alone. A lot of people suffer from depression, not only you. Somewhere around 11million American's each year are diagnosed with depression.

So in short, prepare to be asked personal questions, and be honest about your answers. He won't conclude that your a nut and admit you immediately. Just be honest.

Hope this helps!
And if you need medicine, you need medicine. There is nothing wrong with you.

If you're going to see a GP doctor, tell him you are undergoing severe depression, and ask if he can recommend a psychologist. You need to have a full evaluation by a psychologist from the condition you describe. He/She will ask whether you are having suicidal thoughts, as a routine part of the examination. And don't worry about any doctor thinking you to be crazy, and commiting you. Depression with suicidal thoughts is not uncommon today, and treatment is readily available. Don't hold anything back out of fear! They've heard it all before, on a very regular basis. Get well! It's relatively easy, and painless also, but be HONEST. Trust me, I'm old. lol