While there are some sites on-line where the information is free, the ones I've seen aren't well setup for someone initially learning to read schematics (i.e. they are mostly lists of symbols without much explanation of conventions, etc...). I would recommend getting a decent introductary electronics book, not a college level textbook although some do include this information as well many assume you know how to read. Off-hand, I can recommend some of the books by Forest M. Mims III like Getting Started in Electronics or Electronic Formulas, Symbols & Circuits. There are other worthwhile books, of course, but those two I mentioned are fairly inexpensive and many people find them very instructive.