I think your best bet is to be honest with the sound engineer when you turn up to set up and sound check.
Tell them this is your first gig playing through a pro system and your a little lost with rigging it up.
They will appreciate the honesty and will more than likely keep you more informed as to what they are doing, while they are setting up,
so that you get some ground info. (if theyre not a total dick. most sound guys are pretty cool so long as you don't piss them off)
*all of the following is information I would have wanted to know before my first pro system gig.*
you will need to bring anything that adds a unique flavour to your sound.
if your sound comes completely from your pedal, don't bring an amp, bring a di box and your pedal.
make sure all of your pedal patches are at sensible volume levels(with sudden changes in volume the engineer can panic and drop your channel)
if your bassist doesn't get a custom sound from his bass rig, leave it at home, bring a di.
-you can eq most peoples sounds, and it will have more punch that he is used to anyway.
they may have di boxes at the venue but you will want to be on the safe side and bring your own. -clearly label them as yours
If you dont have a fancy mike then use the venues. or else - clearly label it as yours
don't bring mic cables they will have heaps.
any cables you do bring, again mark them clearly,(fluro colored tape or something.)
if they all look the same they can get lost and you cant just take one of theirs to even it out, they will not sympathize.
if your drummer has a 1000 piece drum kit and only uses 5 for the set then just bring 5 pieces.
they will have drum mics.
you will more than likely not need mic stands.
*bring a roll of duct tape*
bring an electric tuner, i know they are crap, but your guitars will get out of tune in their cases, with transport and heat changes between sound check and night time,
and when its 5 mins before the gig, you cant hear crap backstage.
if its a low level venue with the house engineer doing the work, chances are your foldback mix will be a split of the front of house mix.
this means your music will sound pretty bad to you but out the front of house it will sound great. use it to hear each other and keep your que's
but dont be disheartened by the mix you are hearing.
i'm sure you already know this but have double the material you need for the time slot, adrenaline can chew through a set list in double time.
as soon as your set is over and the lights go down, start packing up. do not let your gear out of sight for a second.
the assistant engineers will already be up there unpacking everything of theirs, because people steal stuff.
get it all to the van, account for everything and then you can relax and have a few beers.
And be nice to your sound guy.
preferably make friends with one who is into your sort of music and genuinely likes you music, when you have jams, invite him round too,
shout him beers and let him play with your gear. (this will set you up for cheaper recording too

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that's all i can think of for now.
sorry if all this stuff is elementary but like i said, i would have loved to hear this before my first pro system gigs so im sharing it with you.
Sometimes I like to use words out of contents