If it's not being recorded (for sale) then just go out and have fun with it. You can't really practice something you haven't been shown, but just get in a space where you get connected with your inner-ear. Think a note, play it, think where it might go, play it. You'll have to rely on your ear when you run out of road map so get connected (as much as possible) with your sense of roots and melody, and your best guess as to where things might go musically.
Still, it'll be great. Sometimes when you're starting off with a handicap - in this case not knowing the songs - there's a point where you go "I got nothing to lose" - and it actually makes you more relaxed, more inspired. And then, if you can focus on the way the song is moving, you can start to anticipate and be creative and have fun with it. Add to that a little adrenaline and you might find yourself rushing downstage and grinning at the audience even if you flub a note. At that point, live, it doesn't matter. Your sense of fun and energy will translate to the audience.
It sorta goes beyond the actual notes.
Have fun with it.
...and let us know how it came out!
