As a slightly beyond beginner, here's what I recommend--
Learn the I, IV, and V chop changes in the the following common keys...
I IV V
G C D
D G A
A D E
C F G
(The letter under I is the key and the next two letters are the IV and V chord for the key).
I found that order is roughly the order of frequency of songs (G most common, second most D, etc) so learn them in that order.
If you can't play 4-note chop chords, play 3-note chords; if you can't play 3-notes; play 2-note chords. Nothing wrong with just playing two-note chords if you have to. You'd do not want to fumble and be late on a beat which brings up the next point.
It's most important to always be on time in rhythm. Sacrifice adding pitch to get the timing right. If you lose track of the chords, just mute all the strings and go "click" until it comes around to a place where you can get back on track.
If you want to spend $20 to get a little before hand practice, you could try Pete Wernick's Bluegrass Jamming DVD. It has some tips to get you started and some super simple songs to get you started.
"Dust off those rusty strings just one more time. Gonna make em shine!" -Robert Hunter