Very important - EQ controls are for reducing feedback, not for 'making it sound better' (EQ is short for equalisation).

If you get low pitched feedback, GENTLY take out the low frequency EQ until it just stops being a problem.

If it's high pitched feedback, ditto for the HF EQ.

It's HIGHLY UNLIKELY that you will need to increase EQ, and you should always start with the controls in the 'flat' position - neither adding nor reducing. This position is usually marked '0'!!

Modern desks have a 'sweep mid' EQ that needs a lot of knowhow to use properly - I've a feeling the desk you'll be using wont have it; if it has, or if you use one that has, there are two controls. One is the 'spot frequency', and the other is the actual 'gain' control.

The trick you can use until you fully understand it is to turn the gain back to around -3dB (the control is usually marked), turn up the channel volume slowly until feedback just starts to ring, then carefully rotate the frequency control until you find the spot frequency causing the feedback.

If you have a graphic EQ - one with more than three faders - avoid the temptation to make a smiley face! It may work (sort of) for disco 'music', put will make a microphone sound terrible, and give you serious feedback problems!

I'm getting too technical, I think - bear in mind that if you need to alter EQ drastically, you are probably doing something else wrong, like pointing a mike straight down a speaker.