So, from an avid fan of Dark Souls, let me give you some helpful tips (None of these are spoilers, nore am I going to tell you about good weapons for right off the bat, figure those out on your own ):
1) If you get invaded by a Greifer, DO NOT play their game. A Common Grief is that they will have you chase them to an NPC, hoping you will attack them, which will force you to kill that NPC. DON'T FALL FOR THAT. Seriously, just leave the Invader, and the NPC, alone. Too often I see people think they have to kill the invader to move on, when really you can just go to the boss and kick them out of your game.
2) (Player Skill >) Equipment > Levels. Levels don't do jack in this game, that's how greifers can grief; they go through the game at level 1, start a NG+, and kill people on their New game with all the gear they acquired. If you have to choose between upgrading your chosen weapon and leveling your Soul Level, go with the equipment, UNLESS you're upgrading your level to use better equipment.
That's really all I have, if other people want to add in more/berate me for either of those, go right ahead. But this is really all you "Need" to know that the game doesn't explain to you.
3) Don't die