Hi Herge - as you say the easiest way is to hire a PA guy to show oyu what to do - although its basically simple.

I don't know what type of desk you have but if its typical you will have an input socket - possibly a jack and an XLR - at the top or at the back of the desk for each chanel. For each chanel you will likely have a pad and a trim pot (also called chanel gain) - using a combination of these the idea is to even out the level of each thing you plug into the desk so that if two things are to come out of the speakers at the same level the position of the chanel faders should be roughly the same.

What you don't want is whatever you're plugging into the desk to overpower the chanel or, on the other hand be so low that you end up with elecrtical noise. The way to go is to monitor the chanel using PFL (with the chanel fader turned down) and gradually bring the chanel gain up until the meters give a good amount of green but not so much that they turn red. If you do this systematically for every input everything should end up at the same level.

You'll soon get to know when to use the pad - mics almost never & things with line outputs (e.g. keyboards) most of the time.
Ray