4:10 a.m. on April 21, 2012 (EDT)
First things first. Do your homework. By that I mean if you are a total beginner, start by doing some reading. I would start with this-http://www.amazon.com/Allen-Mikes-Really-Cool-Backpackin/dp/1560449128
Mike also has a book on ultra light camping (also on Amazon). I have their two books on winter camping and tele skiing and highly recommend their style for beginners.
Then read The Complete Walker, considered the Bible of backpacking. The latest version is the 4th edition. There are many other books, but these two will give you the basics. Mountaineering: Freedom of the Hills is another great book, but more oriented towards climbing.
You will learn far more far quicker from books than by asking a generic question like you have done.
Asking a question may get you a dozen conflicting answers from people you don't know and more importantly, you don't know if they actually know what they are talking about. Anyone who says you "must" buy a particular brand of anything is full of it. For better or worse, there is a bewildering array of gear and clothing available. Don't believe me, walk into your local REI and look around.
Second-don't buy anything until you understand what you need, otherwise you could waste a heck of a lot of money in a hurry for no good reason. For example, if you are just day hiking for the moment, get a cheap daypack, load it up with the "ten essentials" (a basic list of essential items to carry at all times, sometimes bumped up to 14 or so items) and do some short day hikes. You can find this list on many sites.
Everyone else here is saying "buy boots." Why? I have no idea, and neither do you. I know for a fact, they don't know since you haven't said word one about what you intend to do or where.
For now, I would hike in running shoes for starters or buy a pair of trail runners (they look like beefed up running shoes or lightweight boots, depending on the brand and how they are being marketed). Yes, a nice pair of boots is a good investment. I wear boots when backpacking, but for a simple day hike, I just wear running shoes unless I know I will be on sketchy terrain.
If you want to know what the average backpacker carries, look for gear lists; they are all over the net on various camping websites. You need to match your list to your location and weather. For example, I have a gear list that is specific to Yosemite in winter. If I was planning to be there in summer, it would be quite different.
Third-find other people with similar interests, perhaps friends, people at work or through an organization like the Sierra Club, which organizes hikes and other outings for people of all levels of experience. Find a local chapter online.