There are many on-line resources for learning music theory and the skill of sight--reading music.
The following may be particularly useful for people without a music education who become involved with church music or choral singing, and who want to learn to sight-sing or play, or more about how music is constructed.
- Learning to read sheet music: http://readsheetmusic.info/
- Grade one music (the first basic level entry) theory: http://www.mymusictheory.com/grade-1-contents
- A set of quizzes about all aspects of music education: http://www.musictechteacher.com/music_quizzes/music_quizzes.htm, especially tempo markings and basic timing, scales and key signatures
- An on-line game that helps you learn the Bass clef. http://www.studybass.com/tools/bass-clef-notes/
- A resource with more advanced information (transposition, composition, etc.): http://www.musictheory.net/lessons
- A more in-depth resource with chord charts and maps: http://chordmaps.com/
- A guide to ideas of tonality, chord progressions, etc: http://www.tonalityguide.com
Note: none of these deal specifically with church-music issues, because basic musical theory applies in all contexts.