How to find out what loading the server?

Specifically for Apache there is such a tambourine.
\r
1. Open top catch high-load pid Apache
2. Go to server status, calculated there is this pid and see what he does
\r
Option 2:
1. the same thing
2. lsof -p PID will show with what resources works pid

the command htop (install from packages if not present)
or top
this is a system monitor that will show what processes what and how much you eat.

munin set and link it with Apache, nginx and a bunch of all, it is about the system will tell you everything and parishes bots and much more

atop and watch the logs (atop logs load and allows it to "flip through", observing the output of atop at 10-minute intervals).

in the Apache logs, you can enable record processing time of each request
let's see who this time long, so calculate not only the website but also the specific location of the site
\r
LogFormat "%h %l %u %t \\"%r\\" %>s %bb %D" common
at the end of the request processing time in microseconds