Classes are a good idea. :) Most sewing machine stores and fabric stores also offer classes. A good sewing reference book is also really handy. Check used bookstores to see if you can find one cheap. It may sound weird, but I really like how to sew books from the 1950s and before - they're a little out of date, but they're often really good for the basics.

To add to the explanation above, interfacing is used to add strength to a garment and/or stabilize areas (keep the fabric from stretching or deforming.) That's why you use it where you're going to put buttons or buttonholes, because they put a lot of stress on fabric. You also usually use it in collars and on facings around necklines, because those parts of the garment are cut on a curve and are prone to stretching.