Here are a few simple things you can try that might save you a trip to the Apple Store.
Also: Why your iPad, MacBook, or even iPhone might stop charging this winter But first, consider: It might be normal. Laptops get hot, and MacBooks are no exception, especially if they are running the older Intel processors. The newer M1 and M2 Apple Silicon MacBooks in my experience run cooler.
If you’re running a resource-heavy application such as Photoshop, or have a browser with loads of tabs open, then the fans might very well be just doing their job. After all, without them, your Mac could quickly wither and die.
Another heavy load for MacBooks is using external monitors, because these stress the GPU, which in turn means that the fans need to run more often. Not reassured? Keep reading. 

Reset SMC (System Management Controller)

This one tip can fix a whole raft of Mac issues. However, how you go about doing it depends on which Mac you have:
There are a lot of temperature monitoring apps out there, but TG Pro is the most reliable in my experience, and it works on a whole range of Macs – including the newer M1 and M2 Apple Silicon Macs. Another great feature of TG Pro is that it shows you the temperature of all the separate sensors (and there are a lot of sensors, from those built into the CPU to a palm rest temperature sensor) and can warn you if any of the sensors or fans are defective. It’s possible that an application – or maybe even the operating system – has crashed and is going rogue. The quickest way to fix this is with a reboot. Don’t worry, it’s a lot easier than it sounds – oh, and you won’t lose any data doing this!

Desktop systems (iMac, Mac Pro): Disconnect the power cord, wait 15 seconds, plug it back in, and then wait a further 5 seconds before turning the Mac on again.Laptops with non-removable batteries (newer MacBook systems): With the system plugged to a power supply, press and hold SHIFT and OPTION and CONTROL and then press the power button (it helps if you have three arms or an assistant). Then simultaneously release all the buttons before starting the Mac normally.Laptops with removable batteries (older MacBook systems): Disconnect the system from the power supply and remove the battery. Then press and hold down the power button for 5 seconds. Refit the battery and restart the Mac.

Also: How to make your MacBook battery run all day

Reset PRAM (Parameter RAM)

To reset the PRAM you need to hold down COMMAND and OPTION and P and R while powering on the system. Here’s another of those times when a spare arm (or an assistant) comes in handy! Keep holding down the four buttons for about 20 seconds, and you’ll hear the “WALL-E” startup chime or see the Apple logo a couple of times.
If your Mac is overheating, and the problem isn’t a defective fan, then another possibility is that it’s due to dust buildup inside the system. The most effective way to clear this up is to use a vacuum cleaner at the air vents to suck it out.  Hold the nozzle an inch or so away from the vents and only use the suction briefly as you can damage the fans. Don’t use compressed air because that will damage the fans, and all it will accomplish is to blow the debris and detritus deeper into your Mac.