1. Know The Music
2. Bring a Spare GuitarThings can and will go wrong when you start gigging -you need to prepare for disaster. If you break a string in the middle of a song, it may make sense to finish the song down one string. If you aren't skilled enough at string changing to get a new string on in two minutes between songs, however, your best bet is to have a second guitar ready for use. The second guitar is also important if you are playing cheaper instruments - low quality electronics can cut out without notice leaving you up the creek if you don't have a backup. If you don't have a second guitar, then borrow one from a friend. Make sure the the spare guitar is in tune, has a strap, and is ready to grab if needed.
3. Bring (at least) Two Spare Sets of StringsEven if you haven't broken a string in months, you should be fully ready to change a string on the fly. Sometimes rough spots can develop on frets/bridges/nuts, which can cause strings to break quickly, so bring spares (I once went through three B strings on a gig and had to resort to using a high E in its place). In addition to strings, you'll want a pair of pliers and any other tools you use to change strings. Everything should be in easy reach - you don't want to be digging through a bag while the an impatient audience looks on.
4. Bring Something to Tune Your Guitar WithIf it is your first time playing in front of people, you are going to be nervous. One of the first things that goes when people get nervous is their sense of pitch. I've seen many novice guitarists on stage, looking like a deer in headlights, unable to get their guitar in tune. Don't take chances - bring an electronic tuner that you can use easily and regularly. For electric guitars I prefer using tuning pedals, which allow you to tune your guitar without subjecting your audience to the sound of your tuning. You'll also want to make sure the rest of the band is in tune with you... don't start the show without checking this
5. Tape Some Picks to Your Mic StandChances are, if you haven't played for an audience before, you are going to be nervous. This can cause your hands to sweat, which makes it hard to hold on to picks. so, you'll want to prepare for that happening. If you are using a mic stand, put some masking tape on the shaft of the stand, with some of the sticky side exposed. Take some extra guitar picks and stick them to the tape, so you can grab them easily mid-song. If you aren't using a mic stand, just be sure you leave some picks out in a place you can grab them quickly. Some guitarists even tape extra picks to the body of their guitar for maximum convenience.
6. Test Your Cables and Bring ExtrasNothing is more frustrating than malfunctioning guitar cables on a gig. Make sure you test the cables you're planning on using for your show. Try jostling and gently tugging on them while plugged in to be sure the connection is solid. Bring these and a couple extra cables to the gig. If you've got some cables that occasionally misbehave - producing "static" sounds or cutting out entirely, don't even bring them to your gigs a back-ups.
7. Keep Your Hands WarmYour mileage may vary, but I've always found my hands get cold when I get nervous, which is not conducive to playing guitar. When I used to fight nerves before getting onstage, I'd spend a couple minutes holding my hands in warm water.
8. Think About How You're Going to Present YourselfTake a few minutes before you get on stage to think about how you want to be perceived by your audience. Brash? Angry? Mellow? Jovial? Once you've got something in mind, try and play the part onstage. If you are playing in front of people for the first time, you're going to be (and probably look) nervous. By focusing on how you're being perceived, you should be able to avoid looking completely freaked out.
9. Keep Your CoolNot everything is going to go smoothly on your first gig. You may forget some chords or lyrics, you may have technical difficulties or you may have to deal with band-mates running into the same problems. Here's a secret - you can get away with some big mistakes onstage, and your audience will be none the wiser, if you're able to keep your cool. People are much more likely to notice mistakes if they are accompanied by looks of confusion/shock/anger from the performers. If you keep your game face on in the face of adversity, most of your audience will be none-the-wiser that you're playing the song in the key of A while the rest of the band is playing in the key of D.
10. Force Yourself to Enjoy the ExperienceMy first gigs were terrifying - and frankly I don't think I had a whole lot of fun. Don't repeat my mistakes - try and take a moment to step back from the fear and chaos and enjoy the experience of being on stage. This is why you've been practicing so hard!
For those of you looking for some more practical advice on playing in front of people, take a look at these tips on playing for a crowd.