Well to point out a few that I haven't seen covered here, is this: Get yourself a bow, and some arrows. It doesn't matter what class you have, get yourself a bow, not a crossbow, but a bow. A good start is the Short Bow, and a shit-ton of Wooden Arrows from the first undead merchant you come to.
Why? As mentioned earlier, group fights become fairly common later on, and you'll occasionally run into them early on as well. When you get a bow, you can always draw them in for one-on-one fights. Your survival odds will, generally, stay as high as possible that way, though not all enemies will be much easier one-on-one, and some can't be drawn to you at all, depending on where they are.
This will also make some optional boss fights absolutely trivial, by killing large enemies from afar with bug bites. Its slow, and some will say its cheap, but the fact of the matter is, you need every advantage you can get. Sometimes your enemies will fight just as dirty as you can, if not more so.
On another note, do not be afraid to grind for levels, or farm for resources of any kind. Farming for humanity, moss clumps, and titanite is perfectly acceptable, and sometimes needed. Make the Boy Scouts motto your own: Always be prepared. Sometimes this means keeping weapons upgraded for specific situations, or dabbling in Miracles for other options to heal, and trust me, Healing Miracles can be a lifesaver when you need to heal a little more, but want to save your Estus Flasks for when the shit really hits the fan, and it will.
A shield is also a must, at the very least for beginners, but remember: Not all shields will block all damage. Now, when it comes to fire, lightning, or magical damage, no shield can block those forms of attack in their entirety. At most, they can block 80% of those types. However, there are some that can block 100% physical damage, which is the most common, and what you'll be on the receiving end of most of the time, so make sure your shield blocks all physical damage. Not only that, but make sure to look at stability: The higher the stability, the less stamina taken up when an attack is blocked, and less buildup of potential status effects, like bleed, poison and toxic.
Also, keep in mind that while small to medium shields can parry, great shields cannot. They trade better stability and better resistance to damage for higher weight, and a shield bash attack which is, frankly, kind of lackluster. When in doubt, go with a medium shield. Middle of the road is generally best in this case.
Lastly, keep the most important consumable items in the little quick select menu (Down on your d-pad for Xbox, not sure for PC), at all times. Opening up the actual menu, pauses nothing. You are open to attack, and next to helpless. Sure, you can still move, but with the menu in the way, being unable to really see makes moving around almost a moot point.
[EDIT]: Oh, and to address the post above mine, yes, while the Drake Sword is a very good early weapon, in a lot of ways you end up sabotaging yourself for later on, by depending on a weapon that is costly to upgrade, scales with no stats whatsoever, and ends up being very shitty very fast as the enemies get stronger. No sir, it is far better to find a weapon type that you are comfortable with early on, and scale that as best you can. Get used to it, make it your friend, and you will be rewarded.
I myself raided the graveyard by the Firelink Shrine for the Winged Spear practically first thing, and never regretted it.
