Study on the Internet. The Internet is full of websites that can teach you the basics of reading music, more commonly referred to as "music theory." Type "basic music theory" into any search engine. (See References)
Check out the library. Your local public library will have books on basic music theory, including some in the children's and young adult sections that will be even easier for a beginner to understand. Ask the reference librarian for CDs or DVDs on the subject if you learn better by listening than by reading. Once you've learned the basics like note names, check out a book with music in it and practice identifying them.
Practice what you've learned. You don't have to be a great singer to join a church's choir. Regular use of the music reading basics you've taught yourself will reinforce what you know and teach you even more. A choir director should be willing to explain what you don't understand. If you live near a college or university, explore the music department and see whether there are open practice rooms with pianos. If so, take advantage of this resource and try on an actual instrument. Be considerate of music students needing the room for lessons and their own practice --- 20 minutes twice a week is all the practice you really need at this stage.