First thing to do is figure out who and what you are. Age can help determine that. You sound like you ought to shoot for "old salt".
Basically you need to do a
great deal of research - that means
you do it, you don't just ask around and take whatever anyone tells you. Move in to your local library. Look on the Internet and in the library catalog for lists of books about seafaring in the 19th century, there are literally thousands of books on that subject. Read, read, read. Think and talk about it. Read a lot more.
Look for groups that study the subject, there are probably hundreds in the US, it will just take time and effort to find them.
Best last book I read about it is
Two Years Before the Mast by Richard Henry Dana. The most common garb for a seaman at that time and place was "loose ducks and checked shirts". They called their hand-made garments "slops".
Museums are perhaps
the place to see clothing like what you'll be wearing. Check them out on the 'Net, look at the pics. Write to the curators and ask questions. Get thee hence for long weekends of study. Mystic Seaport in Connecticut is Mecca for this stuff. New Bedford in Massachusetts is where you'll find out everything you need to know about whaling. The China and Yankee clippers are a lifetime study all by themselves.
And you
must learn to do fancy knotwork, because you're not going to make a very convincing seaman unless you have tons of things made of knotwork such as fancy ends on the rope that closes your seabag, knotted handles on your seachest, etc. You'll get to know what an "arse"
really is
You could have an impression that few could rival. Fair winds and following seas, shipmate.