Personally, I find that building confidence is very hard to do on your own, as a big part of it comes from your audience's reaction to your playing. If they like how it sounds, you'll most likely be more confident in your playing than you will if they don't.

There are, of course, a few things you can try to do to build a bit of confidence. First off, make sure you can play whatever you'll be playing without looking at your guitar. Now try playing that in front of a mirror, the same way you would prepare for a job interview in front of a mirror. It doesn't quite give you an idea of how it is to play in front of others, but it does give you an idea of how your audience sees you, Which is more important than how you feel. While playing in front of a mirror, try to look as confident and professional as possible. That's how an audience wants to see you, regardless of size (maybe except for family). If you look unconfident, you'll have a hard time convincing an audience that you're worth listening to.

Note, I have played 0 serious gigs, and nothing impressive at school musicals, so I'm in no way an expert on the area. I just know what works for me, and thought I'd share that bit of advice.