If you're putting on a show for your closest friends and relatives, all you need to do is send out an e-mail or make some phone calls. If you're looking to draw your audience from a larger pool, though, you'll need to actively promote.
This basically means getting the word out, and you can do this in any number of ways, from Facebook and social media announcements to flyers around town to newspaper ads. You can notify local-events and music Web sites or ask at music stores in your area if you can put some flyers in their register area.
Whatever you do, make sure the information you provide in your promotional materials is complete and correct. There's nothing like having most of your audience show up on the wrong day.
And finally, in case of rain ...