How can I diagnose a Mac running hot and having high fan usage? My Mac is running hot with the fan cutting in often. How can I diagnose what's wrong?

PS: Some sources mentioned Activity Monitor as a tool to diagnose this, but I couldn't find the application.
As IconDaemon mentioned in their comment, Activity Monitor is by default located in Macintosh HD > Applications > Utilities.

However, since your question seems to really be about your Mac running hot with the fan cutting in often, and since you say you're not proficient with Mac OS - any pointers welcomed, I thought I'd suggest you try starting in Safe Mode and potentially also resetting the System Management Controller (SMC).

Boot into Safe Mode

As a troubleshooting step you can boot your Mac into Safe Mode to see if your issue persists, both during Safe Mode and afterwards when booted normally again.
Follow these steps to boot your Mac into Safe Mode:


Fully shut down your Mac
Restart your Mac
Immediately press the Shift key and keep it down 
Let go of the Shift key when you see the login window (NOTE: If you have FileVault enabled you may need to log in twice).
Take a note of what happens (i.e. is your Mac still running hot, etc)
Now exit Safe Mode to restart your Mac as normal
Again, take a note of what happens (i.e. is your Mac still running hot, etc)


Once you've booted into Safe Mode, let me know how you went. However, if the issue persists (especially if it persisted during Safe Mode) you can reset the SMC to see if that resolves the issue.

Reset the SMC

Your computer's System Management Controller (SMC) is responsible for a wide range of functions, including your Mac's cooling fans, power management, battery performance, and the list goes on. The instructions for resetting your SMC depend on whether your device runs


from a built-in battery?
a removable battery? 
off AC power (i.e. plugged into a power point)?


Since you don't actually specify your model of Mac, I will provide the steps for all three scenarios. Ensure you select the one that applies to you!

For MacBook Pros, MacBook Airs, and MacBooks where you can’t remove the battery on your own (i.e. it's a built-in battery), here’s what you do:


Shut down your computer
Keep the MagSafe adapter (power cable) plugged in.
Press at the same time shiftoptioncontrol (on the left side of the keyboard) and the power button
Let go
Turn your computer back on with the power button.


For MacBook Pros, MacBooks etc that you can remove the battery from, here’s what you do:


Shut down your computer
Disconnect the MagSafe plug (power cable) from the computer
Remove the battery
Press the power button for 5 seconds and release
Put the battery back in
Reconnect the MagSafe cord (or power cable)
Turn your computer back on with the power button


For iMacs, Mac Pros, Mac Minis, etc that only run from a power point in the wall


Shut down your computer
Unplug it from power
Wait 15 seconds
Plug the power cord back in
Wait for an additional 5 seconds
Press the power button to turn it back on


Once you've used the appropriate steps to reset the SMC for your model, report back on how you went.

Intel Mac temperature database

As an aside, you may be interested in the Intel Mac Temperature Database. This will give you an indication of the typical temperature ranges experienced by users of various intel Macs. The list can be filtered to suit your needs.