here is a good article on the circle of fifths
http://en.wikipedia.org/wiki/Cycle_of_fifths
If you find they are in G you could just play open G chord.
If you think you will need to do this again a good way to practice is to try playing along with songs on the Radio or Television. like any other skill the more you do it the better you will get.
Learn the most common chord progresions, and how they are used.
Ask for the sheet music or hymnal, even if you can't read the notes it may have the chords printed above the staff.
Check out the Music theory board, Jerome has lots of great stuff there, and is very good at answering questions.