Good advice from echooos and Danaë.
I would add only, whatever and whenever you give information DON'T LIE.
No one can help you if you're constantly lying.
Journal your questions. Relax :)
So I'm going to see a therapist for the first time. I don't know if I should they'll him the full truth or not since I don't know him or trust him. Should I tell him everything, or only certain things?
Echoos is right. You need to take thing slow so you both feel comfortable. In addition, the first appointment will be a getting-to-know-you appointment.
They will need a little background information from you. I normally ask what style of therapy they will be using. Some ask a few questions and then wait for you to discover the answer. Some therapists are really interactive with their patients. I like to know their style so I know what to expect. Remember, each therapist is different. If this one isn't a good fit, try someone else. Good luck.
Tell him or her what you are comfortable revealing; but don't ever lie---for two simple reasons: They can't help if they don't know what you really want help with and two, they can't do their best work if you are pretending to be something else than who you are.
They will start to figure out the truth eventually and/or refer you to someone else if they determine you are not being sufficiently honest to do good work with them. And that's expensive, time-consuming, and eventually futile for you.
Best of luck---everyone needs someone to talk to that has your best interests at heart and wants to see you be the best you can be.
See, now this is really important to feel comfortable and at ease with your therapist and since trust is a factor which doesn't come all of a sudden and it takes time to build up, you can take your time to get well acquainted with your therapist. However this is equally important to be honest with your therapist and not to give any half-truth or misinformation, otherwise it will be really difficult for your therapist to help you out, which will invalidate the very purpose of visiting him/her.
Ask the therapist about themselves and why they chose their form of work. Build a bond of trust before you divulge everything.
The whole therapy thing is a slow process and I'm sure once you trust your therapist, you'll be able to talk about that got you there in the first place. You want to come away from a session a few pounds lighter sort of speak.