How can JMeter identify what to optimize in a website?

I'm new with website performance testing field and will be using JMeter. After playing with it, I am still having troubles with identifying what to optimize in a website load time?

I'm currently still learning about the load testing - who should I give the performance report to? Developers/Programmers? or Network department? Example of an error I usually get is 502 error or timeouts.

Thanks in advance.