Ok...I'm in a sort of worship band thing at church..we aren't the main one but we play a song or two before everyone gets in there..

I missed the practice last week, so I didn't know what songs we were doing today. Well I got in there and they handed me the sheet music, for this song: http://ultimate-guitar.com/tabs/h/hillsongs/center_of_my_life_ver2_crd.htm ( I play rhythm usually on acoustic). I've never heard the song before, so I just did a d,d,u,u,d,u pattern to it..it went okay but that's definitely not how it goes. The main dude was too busy to teach it to me in the few min beforehand.

So, is there any way to know how to strum a song if you're just given the chords? Or what type of pattern would you do if you didn't know how it went?
You really can't, using a tab. You'll have to have someone show you the song or just figure it out on the fly.

What's psalm 150:4?
Aight..I guess I'll just have to make it to practice haha
thanks though.

Psalm 150:4 (NIV)---...praise Him with tambourine and dancing, praise Him with the strings and flute...
Sometimes you can find tabs that have rhythm built into it, like beats per minute, stuff like that.
Otherwise, just do what sounds good to you.
find the song on youtube and strum along

I've not heard it but hillsong are pretty popular so it shouldn't be too hard
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I couldn't imagine it going TOO badly as long as you played the same chords as the rest of the band. Otherwise, when playing a song on the fly it just takes practice, eye contact with the bassist + drummer for cues can also work to your advantage.
It's just all in the feel. If you've been playing for many years and can play any note you hear, just make up something in your head, then play it. It does require quite a bit of skill but it's a very useful one. I don't know how to teach it or how one can learn it though :s Just time...