I don't have any friends i'm always lonely what kind of doctor can i talk to about this?

Psychologist: You need therapy, a psychologist or an acsw ( clinical social worker) may be able to help.

Lots of approaches: This sounds like a longstanding issue for you, so seeking professional help may be the right approach. Which type depends on what's available, your resources, and what appeals to you. If you're religious a minister might help. Therapy (individual or group) probably is the best approach, whether there's a specific cause or it's just they way you are.

You can improve it: You can improve your life. Eat well, exercise, and present yourself well. Do activities that you like and smile and meet people. Ask for their phone number and become friends. You can join a gym and take group classes, find a job with nice coworkers, go to religious services, join a meetup.com group. Keep a good attitude and try. Keep a smile on your face. Good luck to you, believe in yourself.