What I need help with is figuring out what the treble, mid, and bass are used for as the worship team sings. How do I know when to change anything?
The best way to learn is to see "what this knob does". Of course, you'll want to do that during rehearsal, and let the worship team know ahead of time that you'll be making some drastic tweaks to the sound. Just make sure you know where you started from so you know how to get back to normal.
Just as a quick primer, I'm assuming that the "treble, mid, and bass" you are referring to are the controls above the channel fader.
This is an equalizer for that channel and that channel only. I'll further assume that there is a master EQ or signal processor that has been properly set to provide a smooth frequency response from the system and speakers in that particular room. (Once set, you should never have to adjust the master EQ unless something in the system, the room, or position of the speakers changes.)
The channel EQ allows you to fine-tune the sound, adjusting for frequency response of a microphone, the position of the microphone relative to the source, or to smooth out the vocals. Sometimes, you might need to adjust it to fight feedback. The other folks on here will be far better able than I to describe the effect each control has on the sound. If it sounds really bad, zero them all out and start over.
I'll leave you with this tip: most feedback/ringing is a lower frequency than you think it is, until you really learn the pure frequencies. Check out
SFT - Simple Feedback Trainer (it's free).