If I were just getting started, knowing what I know today, I would start with the sleeping bag. And where and when you hike will make a big difference in how warm a bag you need. Get a good bag that will keep you warm, but don't by an arctic monster for the AT in July!

Backpacks? I'd get a cheap, used, external frame pack at a thrift shop or garage sale. I see them all the time for under $15. And they'll work just fine for a few days in the woods.

I agree that you can share a tent. And if you are not worried about bugs, a tarp and a couple of poles will keep you dry for almost no money at all. $10 or so.

Let's go hiking!