Okay, so I see a lot of people on here that seem like they are getting into the swing of things on Dark Souls. Let me offer some advise to anyone who wants to listen to it :). Okay so first and foremost, any player can do well as long as he knows the rules of combat. Soul level 1 play throughs are possible with correct item raising etc. This illusrates the importance of combat tactics, learn your parrys and dodges!
Secondly, decide on what type of character you want and stick with it. Balanced characters are nearly impossible if you want to ever beat an invader, or invade for that matter. Make a DEX build, STR, or INTE respectively, FAITH is another good class choice if you incorporate divine/occult clubs, the same is true of intelligence builds with enchanted things. These give you the capability of killing things at close range as well as kicking butt with sorceries/miracles. CHOOSE WISELY!
To go along with my previous point, make your choice and wear the according armour. If you are a light build wear light armour unless you have excessive endurance. Medium, medium armour etc. The same is true of the weapons, which should fit into place anyways, seeing as most scale. If for instance though, you love faith, but want to cast sorcery, you can grab the tin dark moon catalyst. Velkas talisman is the reverse, for intelligence users that want faith.
In PVP, do not be a jerk, people will tend to respect you if you respect them and don't backstab fish or spam wrath of the gods etc. Never ever, ever, ever sit and spam the regular attack at an invader, any compitent person will have you parried and dead in seconds. Use your environment, gain separation if you need to, use drop attacks, kicks, but avoid aggroing coms.
Lastly, take the time to explore everything, and I mean everything. Always spend souls when you can, and never hoard them unless you are very experienced/need to save for an item. This game is unforgiving to the point of being torturous to people who do not manage humanity or souls well. AND ABOVE ALL ELSE STAY CALM AND COMPOSED. Most deaths in Dark Souls will happen when you panic, just be chill and enjoy the ride and absorb the amazing story that is Dark Souls.