Due date? Help

Hi everyone, so I just found out I'm pregnant. I took two pregnancy test, one last night and one this morning both bright and clear bfp! I'm so excited but I don't know when my due date is. I've never had regular periods, they were always all over the place so I cant even go based on my last period cause that would probably be August and I know I wasn't pregnant in September cause I tested. Is there any way to tell how far along I am or can the doctor tell? My periods would always be 2-3 months apart. Help please! :/