wow, that's a huge question.
The most common pattenr of Jmeter is likely to slowly crank up the number of users over time, until the system hits a "wall" - that is when messages stop getting through. start there. Also be cognizant of your machine and bandwidth; a remote laptop running Jmeter over a slow connection might be bottlenecked by the laptop or the internet connection. So check both the jmeter logs and, more importantly, the server response time using a different mechanism, likely the server logs.