Stand up straight with your shoulders back. Place the guitar strap over your head and rest it on your left shoulder, then realign your posture. You might have the tendency to slouch or curve your body forward in order to pick the strings or look like you're cool. Stand up straight anyway. If you don't, your vocal chords get crimped and you lose breath support. Of course, if you're not singing, you can do whatever you want with your posture.
Make sure the guitar rests comfortably near your abdomen, at about hip height. This is the best position to play from because your arms don't need to stretch or bend too far. You're not playing a ukelele; let your arms remain relaxed.
Keep looking forward instead of looking at the bar to make sure you're hitting the chords. If you're very new to guitar, you might want to wait to perform on stage until you're confident with strumming and can look up while playing. It's good to check in every once in a while, especially for those hard-to-reach chords, but looking down the entire song means you need more practice.
Stay in the same position as best you can. Nerves make people jittery and want to move. But moving increases the chance of you bumping the mic stand, hitting your hand, getting the strings out of tune or knocking something over. Become ultra aware of the space you're confined to and plan your moves ahead of time.