Scratch a dozen dedicated hikers, and you'll find a dozen different opinions about what kind of gear you absolutely have to have on the trail. Some people routinely overpack -- they're the ones carrying a sleeping bag and tent on a short day hike, just in case. (There's nothing wrong with that if it makes you feel comfortable and prepared, and as a side bonus, you'll be totally buff by the end of the summer.)
Others eschew basic comforts like insoles and the fingertips on their gloves, all in the interest of shaving a few ounces off their load. Of course, most of us fall somewhere in between those two extremes. But the one thing we all have in common is that, unless you're ready to plunge into the wilderness in nothing but your underpants with a knife clenched between your teeth, there's a core list of essential gear you just can't do without. If you're gearing up for your first hike, these are the items to focus on:
No matter what kind of stuff you're going to carry -- even if it's just a bottle of water -- you need a place to put it. For short hike in a mild climate, the smallest pack I'd recommend would have about 1,000 cubic inches (~16 liters) of carrying capacity. To put it in perspective, that's about the size of a kindergartener's backpack -- just large enough to carry sunscreen, a snack, a guidebook, a small bottle of water, a packable rain jacket, light gloves, and a lightweight hat.
I have a lumbar pack (a small pack that secures around my waist -- no shoulder straps) that's about this size, and I absolutely love it for short hikes because my back never gets sweaty.
However, if you plan on hiking with any frequency or going longer distances, I recommend a larger pack -- 1,500 to 2,000 cubic inches (about 25 to 33 liters). The extra space means you can carry more water, more snacks, and more layers of clothing, plus a rudimentary emergency and first-aid kit.
Ultralight hikers can fit everything they need for an overnighter into a large daypack, but most people will be comfortable with a pack that fits at least 3,000 cubic inches (50 liters) -- go larger if you're carrying a large tent, sleeping pad or sleeping bag, or packing for more than one night.
I also recommend a pack with exterior water bottle pockets, an exterior bungee or cinch strap for layers, and external attachment points for hiking poles. You don't have to use those external attachment points, but having them gives you the option of freeing up interior pack space for other items.
Even barefoot hikers don't really go hiking barefoot -- they wear wacky toe shoes like Vibram Five Fingers. (Actually, I don't think they're all that wacky -- I like them!) And even ultralight hikers know they won't get far if their shoes or boots fall apart on them. As a general rule the farther you're planning on hiking, the sturdier your footwear should be.
You can even get away with tennis shoes or cross-trainers on a short hike; just be aware that the lighter your shoes the less ankle support you'll have, and the more you're going to feel every rock or root you step on. No matter what you're hiking in, the most important thing of all is that your hiking footwear fits.
Clothing is more than a style statement -- it's a form of mobile shelter. Again, everyone's going to have their personal preference here, but you can almost always count on needing a weatherproof jacket and, on longer hikes or in situations where you know you'll be far from easy shelter, weatherproof pants too. The colder it gets, the more extra layers you'll want to have along.
Depending on your environment, you might be able to survive as much as a few days without water -- but turn up the heat (or cold), add exertion into the mix, and if you're not drinking enough water you could suffer uncomfortable symptoms of dehydration, or even heat exhaustion or heat stroke.
So, how are you going to carry the water you need? I'm fond of Nalgene bottles because they're the closest thing to indestructible. If space is limited I also enjoy plastic water pouches that roll into a tiny bundle once they're empty (some of the smaller pouches fit nicely into the inner pocket of a jacket).
You can also slip a hydration reservoir (read: a big plastic pouch filled with water, with a long drinking hose attached) into almost any pack. Feed the hose through the small hole most new packs offer for this exact purpose, then clip the drinking hose to one of the pack's straps or the front of your shirt -- you're can sip water through the hose without having to stop to fish a bottle out.