First of all, you should be practicing in all the keys and not just C. Start to apply what you know in the key of C to the other 11 keys so you won't get stuck in just one key then have to transpose when other people sing in other keys.
Next, you should start with easy songs that have simple chords. Songs like "Thank You Lord" or "O Come Let Us Adore Him." You know slow songs that aren't too involved and then build from there.
Last, be patient cuz it ain't all gonna happen over night. It will take years of practice.
Tip: Just playing one song is not enough if you want to become a better musician. Progressions is the key to learning songs faster. Most of your congregational songs use the same chords, same beat, same everything. Instead of just playing the song, it would be better to learn the pattern so you can play every othere song that sounds like it.