What happens if i tell my doctor that i think about killing myself sometimes?

That depends: Your doctor may refer you to see a pyschiatrist or a psychologist. If you have an imminent, strong desire to take your own life - then psychiatric admission is a strong possibility. It is important to tell the doctor if you feel suicidal now. Do you have a plan? Have you rehearsed the plan? Do you have a reason to live? Do you have a support system? Do you see no other options other than suicide?