Research self-publishing companies. Christian-specific companies may feel more comfortable to you, but choose the company that is the best one to meet your needs. Some Christian self-publishing companies may be able to help advise you on how to market your book to a Christian-specific audience. Look at the packages offered by each company, their pricing structure, and how much control you maintain over your own book. Make sure you aren't signing your rights away.
Ask around for other people's experiences. Most author services companies are experts at selling the dream of being a published author and holding a book with your name on it in your hands. Don't sign any contracts or sign up for any services before you've talked to some of the authors already in that company's catalog to find how how happy they've been with their experience.
Avoid unnecessary charges. Register your own copyright to save money. It's not difficult and you only have to go to copyright.gov/register and follow their directions.
Pick a company that let's you buy a la carte. If you can get editing or cover design better elsewhere for a less expensive cost, you should. If you get your cover art through the company make sure you have the rights to use that art if you ever choose to use a different company or printer.
Sign up for a company and follow their directions for file submission. Author services companies tend to be the most "hand-holding" method of self-publishing, so if you're confused at any stage in the game over a company's procedures, they'll walk you through it.