Study the basics online. There are several websites that explain the basics of music theory for free. Take your time as you read through the music theory website you choose; don't try to master it all in one afternoon.
Practice reading music. Borrow a book of hymns from your church or a book of folk songs from the library. Learn to identify what key each piece is in (G, C, D, for example), and name each note by letter, what kind of note it is (quarter note, whole note) and what kind of rests the composer uses. Continue to practice until you can identify these things quickly; they are the building blocks of music theory.
Work on an instrument. The easiest instrument for learning music theory is a piano -- all notes are laid out in order of pitch. If you do not have access to a piano, or even a toy piano or portable keyboard, check with your local college's music department. Student practice rooms are often equipped with pianos. Take your music book and try to play a simple tune from the written music. Be mindful of students who need to use the practice space and limit your practice to 30 minutes or fewer.