The Knight is the best choice for a beginner, because attacks that nearly kill you and panic you as lighter classes can almost be ignored. On top of that, the knight is very hard to topple he/she can take a full horizontal hit from the biggest bosses and still be standing (if using a shield). People who say other classes forget that when someone first plays the game they don't know any of the enemies patterns or how to dodge attacks. Knight lets you learn that stuff while being forgiving in the meantime.
You should level up roughly according to the stat distribution you start with (+2 to vitality, then +1 to str dex and faith, + 1 Vitality, +1 end, res, attune, +1 vitality, +1 str, dex, + 1 vit, then +1 all). You can skip int if you want, but if you don't you will eventually have access to useful low level spells like magic shield. You can start leveling up your knight armor or elite knight armor once you find it, but really you want to rush for stone armor.
Save up and buy the Crest of Artorius from Andre for 20k souls asap. Besides the NPCs who attack you in there giving you avg 2k souls each to pay you back, you can cross the bridge and go under it to get Stone armor. You will also need Havel's ring to wear it, who is in a tower on the other sight of darkwood by the hydra. The key to the tower is after the moonlight butterfly. For the butterfly reverse hollow and check the bush under the stairs to it's fog door. You will find a summon sign for Witch Beatrice who will eat the butterfly alive.
The armor combined with your high HP will basically allow you to laugh off most attacks in the game, and you still have enough maneuverability to dodge, block etc. Good shields include Grass Crest (stam regen) and Eagle Shield (stam conservation). You'll eventually want to get the stam regen ring, and replace your armor with fully upgraded giant armor later in the game. For a few bosses/enemies (such as (ceaseless, bed of chaos, basilisk), armor doesn't help much so you may want to put something lighter or specialized temporarily.
Halberds make good weapons for this slow class, specifically the vanilla one cause it has a spear like attack and a sweeping attack. It has long range so at least if you miss you have time still before they reach you to recover, and it can sweep fast or multiple targets. Your strength won't get that high very quickly, but you don't need much for these mid-high damage long range weapons. They are not as unwieldy as greatswords and such. I usually rush to vamos to start it on the fire path asap. Other people might prefer gravelord greatsword, zweihander, which are also low str req long range weapons. (running attack is good on zweihander for long range, GLGS has a long stab r2)
Pvp I have had good results with the class, the most annoying foe are Thief types connecting from Poland as they tend to back stab you from 20 feet away while facing the wrong direction. When this happens I tend to just use 2h sweeping attacks and hope you can damage them enough to kill or panic them.