Depending on your playstyle/build; always be blocking/dodging/parrying when appropriate. Timing is key, observe enemies and bosses attack patterns, don't get greedy with an opening, if you can attack twice, attack twice, don't squeeze in a third. You can summon other players for tough encounters, some boss fights also have NPC summons to give you some breathing room. Search every nook and cranny, there's lots of hidden items. Look up what weapons boss souls make, if the weapon is not relevant to your character (i.e. A strength weapon on a dex build) consider breaking the soul for extra souls to level up. You can join the Blue Sentinels, Brotherhood of Blood or the Dragon Covenant if you want to focus on PvP, Brotherhood and Dragons are good because there's no cost to engaging in PvP, unlike the Sentinels, which require tokens of fidelity to fight others.