In the last few years, I've found certain websites, programs and books to be invaluable to what I do on Sunday mornings. These are a few ideas on what to think about for doing this job, and also the websites that I've found the most helpful:
The sheet music I receive can run the gamut of chord charts and lead sheets, to actual written-out music. Some examples from yesterday's service: chord chart - lead sheet. If I want something with more notes, I've found PraiseCharts.com to be a great resource for Contemporary Christian songs. Every church adjusts the music as needed, so expect to cut and paste, repeat, or transpose as the singer or congregation requires. If PraiseCharts doesn't have it, often MusicNotes.com does.
If all else fails, you can create your own music: the notation programs currently used a lot are Finale and Sibelius. Its just a really handy skill to have, also, so I suggest learning at least one of them.
Often the written out music in the hymnal is four-part harmony, and doesn't function well as an accompaniment. Sometimes I research a hymn to see if I can find a style I like better than my version. Some hymns are often swung or etc, and it isn't indicated on the music. Between Youtube.com, itunes and Amazon mp3's, I'm usually able to find what I'm looking for. With current stuff, you really have to listen to it just to get the sound and feel right (there's not much to go on in a chart).
Matching music to text
Part of the job of the church pianist is to help make the service flow as a unified whole. One way to do this is by choosing music that reinforces the day's scripture reading (You can look it up in the Online Bible) or the day's sermon. I've tried looking online for either a 'scripture index to hymns', or a 'topical index to hymns' for ideas of which music to choose - but haven't had much luck (at least on the free sites). It's best just to stick with your church's hymnal, which should have some kind of index that will help.
Prelude/postlude solo piano music:
Mark Hayes stuff is pretty standard
Fred Bock is simple, yet pleasing (The arrangements that I like, such as On Eagles Wings, Here I am, Lord, Hymn of Promise, etc are all in this book)
Anything classical is typically fair game, especially literature everyone has heard at one point in their lives (Claire de Lune, anything by Bach, etc)
If you have a singer, art song can be used as well. Try:
Aaron Copeland's Zion's Walls, At the River and Simple Gifts
Lee Hoiby's The Lamb and The Shepherd