Q: I have a 2013 iMac, 27" desktop.  Every time I turn it on now the cooling fan starts running.  How can I stop that?

First you need to check for any hardware failures. Please use this Apple article:

Using Apple Diagnostics - Apple Support

to run Apple Diagnostics on your computer and reprot back here if there are any findings.

You can also do an SMC reset when seeing odd fan behavoir:

Resetting the System Management Controller (SMC) on your Mac - Apple Support

Runaway applications can shorten battery runtime, affect performance, and increase heat and fan acti…

Ways to help make a slow Mac faster

17 Reasons Why Your Mac Runs Slower Than it Should

Slow Mac Performance? This Article Solves It!

Fix slow start-ups in OS X | MacFixIt - CNET Reviews

How to fix slow shutdown and startup times. | MacTip.net

6 Easy Tips to Speed Up OS X Yosemite on Your Mac

Avoid using any third-party software that claims to clean up your computer. Usually this software does more bad than good. Furthermore, you don't need it. Note that all computers will become slower over time even under normal use. Experienced users typically erase the hard drive and do a clean install from scratch at least once a year or whenever installing a major OS upgrade. Of course doing so also means you must maintain regular and multiple backups.

Add more RAM or cut back on the number of concurrently running applications and utilities. Remove unnecessary software such as anti-malware and software that promises to clean your Mac. Check for runaway processes: Runaway applications can shorten battery runtime, affect performance, and increase heat and fan acti… Also see:

Pre-Mavericks

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 (>=70,) 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.

Mavericks and later

Open Activity Monitor in the Utilities folder.  Select All Processes from the View menu.  Click on the CPU tab in the toolbar. Click twice on the %CPU column header to display in descending order.  If you find a process using a large amount of %CPU, 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.

The Ultimate Fix

Backup everything, erase the drive, reinstall OS X, and restore your data from the backup. Reinstall third-party software from original media/scratch.