Press CTRL + ALT + DEL, whether you're on the welcome screen setting or not depends on whether you're receive Task Manager straight away, if you don't, click Task Manager.
Once you have Task Manager opened simply go to the Processes tab. From there you can sort which application is hogging your resources. I'd advise to firstly, sort by CPU usage, and end the process of anything that seems to be taking high than 80, as long as it's not a system process. Next, I'd advise you to sort by memory usage. Simply, again, end process of anything taking too much.
If you're unaware of how to sort the processes, simply click the header; click the CPU field, for the CPU, and click the Mem Usage field for the memory.