Easy. Whenever you die, ask yourself: "What could I have done to avoid that?" A huge majority of deaths in Crucible are avoidable mistakes, such as wandering off by yourself, not being ready for that charging shotgunner coming around the corner, and overextending into exposed areas like sniper lanes or into the crosshairs of that guy charging his Bastion. As a fledging PvP player, the best thing you can do is stick by your teammates and provide cover fire. You will get kills and keep your party alive.