More and more coin dealers are offering currency these days, and you'll almost always find one or more currency-exclusive dealers at any decent sized coin show. Look over the material, just about any dealer worth his salt will be happy to show you anything you want to see, and answer your questions. If you run into an obnoxious dealer, just walk away and take your business somewhere else. Get a book. A great book for starters and seasoned collectors alike is the Standard Catalog of US Paper Money published by Krause Publications.
Tremendous bargains are available on the Internet and the Web.
But there are sharks in these waters too, my friend. When you're ready to buy your first bank note, DON'T buy it online. Go to a coin or currency show. You'll get lots of answers, I guarantee it. This is a fun hobby.