Everyone who has heard of Dark Souls also heard the line 'git gud' by someone who plays the game. Hopefully, with this guide you won't be called a scrub again.
1. Most bosses have a special move or something that catches you off guard, always expect a move they never used before and get used to it.
2. Dodging is much better than blocking because it allows you to avoid all damage and avoid attacks that are unblockable. Shields are still useful though, if you can't dodge in time or have a powerful enough shield and a weak enemy then don't be afraid to turtle.
3. Start PvP early. It helps a lot. You don't want to be the guy who never invades in NG just to try it in NG+ and get your ass kicked by more experienced players.
4. Don't put all of your points into vigor. That is the noobiest thing someone can do in a Souls game. Sure it increases your HP but only enough to last 2 more hits before getting stomped into the ground.
5. Try to lure out enemies. Rushing in does no good and will only get you killed. You are not Dovahkiin, you are not a hero, you will die a painful death. Always try to fight one at a time.
6. Joining a covenant helps a lot. It can grant you special benefits that you wouldn't get alone.
7. Be careful with your stamina. You have a stamina bar, you better use it wisely. If you run out of stamina you are open and can't block, dodge, or attack. This is especially relevant in PvP where a person can guard break you if you lose all of your stamina while blocking.
8. Watch out for overpowered builds like the giantdad, they will fuck you up quick. Luckily, they are often guys who can't play the game on their own and have to use what's closest to god mode without hacking so if you're skilled enough they should be beatable.
9. Hacking is common in Souls games (at least on PC). Luckily, like the giantdads they are unskilled players who can't stand up to an experienced player and not get stomped. The only way I've found to kill these bastards is to drop them off the map, that is usually an instant kill even with hacked health. Don't forget to report them afterwards.
10. Don't get angry for any reason. Rage clouds your judgement, and Dark Souls is hard enough with full focus, so don't get angry if someone chugs an Estus in front of your face, no matter how hard it is not to.
If someone feels like I should add something then tell me.