Yes, you should have a regular physician. There are different options and am glad you are asking!
Your brother: at age 12, he should still be seeing a pediatrician or at least a "family practice" physician.
You: as a female adult, you can use your gyno if you want. Insurance companies generally allow that for females. Otherwise, a "family practice" or "internal medicine" or "general practice" physician would work.
Now, as to how to find one. You need to find out who your insurance carrier has listed as approved for them. You can usually find that on their web site or by calling customer service. Then, you need to ask around to friends and family if anyone has used any of the people on the list and their opinion of them.
There are other things you can do but these are the basic steps that you can start with now.
As for what they do, generally you see your PCP for annual physicals (to just keep check that you're doing OK), colds, flu, upset tummies, the general day to day maladies that strike all of us from time to time. If something more complicated should arise - allergies, asthma, heart problems, etc - he/she can refer you to a specialist or suggest other things.
Hope this helps. Good luck!