The fan runs continuously. If yours is running faster than idle then:
Open Activity Monitor in the Utilities folder. Select All Processes from the Processes dropdown menu. Click twice on the CPU% column header to display in descending order. If you find a process using a large amount of CPU time, then select the process and click on the Quit icon in the toolbar. Click on the Force Quit button to kill the process. See if that helps. Be sure to note the name of the runaway process so you can track down the cause of the problem.
OK, then you need to determine how fast the fan is actually running, and how hot the computer is getting and why.
I would suggest you start by installing some system monitoring software. I recommend using iStat Menus 3.23 to provide menubar monitors for lots of information including information on fan speed and temperatures.
Oh, and yes you can quit Activity Monitor for now.