Always ask yourself "can this fight bring us any value*". IF yes - fight, if no - leave. BAM now you know how to play HOTS ;-)

value: guaranteed kill without any friendly deaths, baited ults before objective, big AOE damage on most of the enemy team forcing them B just before objective, Boss kill or steal without wiping there, taking camps in right time* etc.
** right time: just before objective, to create structure killing waves if they have poor wave clear, to force enemy team splits

Hots is very simple game mechanically with rather low skill ceiling