The internet has revolutionised promotion, and there are many places online where you can spread the word. Specialist music forums and local listings sites, maybe your blog if you've got one and of course Facebook.
However the most direct and time tested method of gig promotion is putting up posters and giving out flyers. Though flyposting can lead to an unwanted run in with local law enforcement if your not careful, there are plenty of legal public places to put up posters.
Presenting the 'who, what, where, when and how much' in a clear and eye catching way can be done with the minimum of fuss. You could splash out on colour, a fanciful design and large print runs, but for most non-commercial events, this will be all you need:
You may decide you need tickets as well, a different ratio of posters and flyers or any number of other bits and pieces. Get in contact for more info.