Leeneia, free resources you can use:

Many local papers have a "This is what's happening around town" section. Call the local paper where the concert will take place and find out. Many will plug the gig for free.

Ditto whoever the local radio stations and TV stations are.

You can get someone who is a bit more savvy than you with computers to produce a decent poster. There are businesses in Kansas City who will display the poster for a few weeks. They don't charge for that. Stick with 8 1/2 x 11" paper.

Colleges and universities in the area might allow you to post to their bulletin boards. Don't overlook student unions. They can be helpful in that regard. Also college radio stations.

The church itself could get some posters up. I'd stay away from stapling them to telephone poles, etc., because many people see that as a very small step up from vandalism.

Regarding the poster: Make it easy to read and ensure all the pertinent info is on it. I saw a beautiful poster once on which the date of the performance had been omitted.

I'm sure you will receive more ideas as the day goes on.