A useful tip: When performing and in doubt about the fingering of a certain chord (perhaps you see a chord like Cmaj7#11), opt to play just the root and fifth, or just the triad (C major here) rather than guessing at how the chord is played; worship music is more about creating an atmosphere, and playing some wrong chord tensions is worse than not playing them at all. Less is more sometimes.

In other cases, it is appropriate to spice up simple worship songs a bit. On a song as simple as "Lord I Lift Your Name On High", where the verse is G, Cadd9, D, Cadd9, you can spice things up a bit using some chord extensions. You can make the G a Gadd9, the C a Cmaj7 (though sparingly), and maybe the D a Dsus2 or a Dadd9,11 (x, 5, 4, 0, 3, 0 - like a C major fingering slid up 2 frets, neat chord when used correctly).

You should be well versed in theory before attempting this, though... practice at home and use what you learn at home in church. After doing this a while, you can approach a song you've never played before and substitute and extend on the spot - but that's after years of practice.

Hope this helps. Keep up the work, and inform us of progress! Have fun, and worship and everything will turn out fine. That's the key, if you don't worship while playing, the people you are playing for won't either. Remember, you're not out to impress anybody, you're trying to create an atmosphere of worship.