You have to learn the fat guys timings. He is vulnerable to backstabs. You can also run past him, he won't chase you and he doesn't drop anything special.

I'm not sure which knight you are referring to... the sword and shield knight you should learn to parry. If this is the problem, go back to the Tower Bonfire, run downstairs, and practice parrying. A buckler shield is highly recommended.

The spear and greatshield knight I would recommend using a longsword with stance weapon art. When he is a meter or so out of the range where he can jab you with his spear, wait for him to put his shield aside and use your R2 weapon art. You will charge towards him and thrust upwards, possibly take a single spear stab from him, but you will stagger him. Spam R1 attacks at him until he puts his shield up again. Then quickly use your WA R1 guardbreak attack on him. This will knock his shield aside and you can walk up to him and riposte him for a large amount of damage.

If you're unfamiliar with how to use weapon arts, practice them at the firelink bonfire. Some shields will let you go straight into weapon art mode, some shields only have shield-bash mode, and other shields only let you parry. You should carry both a weapon art shield and a parry shield.