If you have an index.html or ... , it would be
www./mydomain.com/subfolder/. Of course you could call another file say
www./mydomain.com/subfolder/myfile.html.
You said you have been reading lots. Where are you getting your reading material and what is your business plan - galleries, free sites, etc? Maybe I could hook you up with some links that might answer some questions for you.
You might also get a quicker response to your questions if you post them on the newbie board.