Start by picking up a good, short introductory book on backpacking (I'd recommend Karen Berger's Hiking and Backpacking), and you'll get a lot of your basic questions answered. You don't need to study it closely, just read the parts about getting ready and gear (for now, skip the parts about where to hike, first aid, and other stuff you'd need to know if you were going by yourself.)

Will your hiking partner be supplying the cooking gear and water filter? If so, don't worry about that.

You will need a tent (unless your friend has a 2-person tent, is willing to share, and you don't mind it possibly being a bit snug for two.) You will need a sleeping bag and a sleeping pad (self-inflating or air mattress, unless you're used to sleeping on a thin closed-cell pad.) You will need a cup and spoon (and maybe a bowl, if you don't eat your freeze-dried meal from the bag); you can skip the cup if you're willing to drink just water - then you drink directly from your water bottle. A small pocket knife, a headlamp, appropriate clothing (depends on temperatures), rain gear (weather dependent), and you're pretty good to go.

Food - keep it simple: oatmeal or granola for breakfast, snack food (jerky, granola or energy bars, dried fruit, maybe hard cheese or cookies - no candy bars, which melt if it's hot) for snacks and lunch, and a freeze-dried meal for supper. And remember, lunch is often just a bigger helping of snacks. Gourmet? No. But for one night, it will be fine - and it doesn't sound like eating well is a priority of the trip.

For a pack, a 50 liter pack will probably work (definitely no larger than a 60 liter. Might it be a bit too large for this trip? Maybe - but it will be good for backpacking trips you might take later - this is an easy sport to get hooked on. Your 22 liter pack won't work - not large enough, and probably not a good enough suspension to carry the weight you'll be carrying. (Point of etiquette: if you are sharing gear, you should offer to carry part of the shared gear - another reason for the larger pack.)

How are your boots? Comfortable boots (or, my preference, trail shoes) and good socks are critical. If you already have good comfortable boots, what kind of shape are they in? Don't want them falling apart on the trail.