I had that same initial hurdle you have when I started a few months ago.
Once you do your first gig.. as soon as the first band's set starts and things start to flow, you'll feel that weight being lifted off you, and being replaced with some pretty intense adrenaline. The mix is happening in real time, there's no second chances, so you've gotta get it right. Yet, at the same time, keep in mind a majority of live sound engineers have no idea what they're doing, so if you screw up in some way at a small gig, the crowd will be more or less used to it.
Things I can suggest:
Get a program called Simple Feedback Trainer and learn all the 31 band graphic frequencies by ear. Know what to pull out when it presents itself as a squeal.
Carry a kit with you:
Masking/electrical tape
Gaffa tape
Black markers (multiple, in case one fails)
Torch (extra batteries!!!)
A reference CD
Working microphone leads
Cable tester (you'll be surprised how much inhouse shit doesn't actually work)
If your budget allows it, also try to carry a few extra mics with you, in particular a bass drum mic (most venues have shit inhouse gear and as long as your kick sounds shit hot, you can build the mix around it).
I read a book called Live Sound Mixing which helped a lot. Written by a local Australian. I'm sure you'd have an equivalent wherever you live.
Try to go out and assist at a few gigs before you attempt it solo.
That's about all that comes to mind so far.