Bipolar?

My name is Kaitlin and I'm 18 years old. I have been under the impression for a while that I might be bipolar. But i don't want to make any assumptions before I'm actually  diagnosed. How does one go about getting the help to actually see if I am or if it's all in my head? Cuz i want answers. I've always been really emotional and will go from happy to really sad in a matter of seconds. It might be hormones but honestly i'm not so sure anymore. How does one "get help" to see if I really am bipolar? Who do I talk to? A doctor? Therapist?