Mushroo hit it on the head. You already know enough to play with most cover bands. Following the chords and using the root, fifth and eight will always get you asked back.
Little theory needed for the times they change things up and you have to play in another key, etc. will help. Some of the things I did right at first follow:
I pay from fake chord sheet music.
We all do so, the advantage of this is we all have the same sheet music on stage - except when jamming - so we all are on the same "page". I come armed with sheet music that we all will be paying from - even the solo instruments. Yes its called fake chord as we all follow the chords and the solo instruments - fake the
melody.
OK the fat lady comes out of the dark and askes that we play her song in Bb.

What can you do
Use a capo and if you are using scale degree or Nashville numbers (R-5-8-5) just move up or down the neck with your major scale pattern to where the 1 in the pattern is under the Bb note on your fretboard. Move the pattern and place the root note and you will be playing in the key the fat lady wants.
If you have time transpose the fake chord. How?
Write out in the margins the G chord progression your sheet music is in - yes on the sheet music - then write out the C chord progression under it.
G, Am, Bm, C, D, Em, F#m7b5
C, Dm, Em, F, G, Am, Bm7b5
All of the old G chords become C chords, all the original C chords become F chords, etc.
Now take a copy of your old key of G sheet music (keep what you have) and with a black marks-a-lot and mark through your old fake chord sheet music so it now has the chords in the key of C. Now you have this song in the key of G and C... Put both in your gig book.
Mambo4's link - basic theory PDF - is a great learning tool - spend some time there.
Theory is like eating a great big bear. Best down one bite at a time.
Have fun and good luck.