Im new to HTML, CSS and web designing and so for the term PIXELS. I found that in CSS, we fix font size and everything in pixels, not in cm or mm so Im confused. I would like to know how any cm is equal to 1 pixel and how to calculate pixel.

If you look at your PC screen REALLLY closely.

You will see tiny squares ..

These are pixels.

So the size actually changes from PC to PC.
However 1 pixel = 1 of those tiny squares

AND Your monitor will have a specification of pixels(dots) per inch (DPI)

The standard for windows is 96 dpi and Mac monitors are 72 DPI.

It is the job of the screen driver to calculate relative sizes to exact pixels.

Yes, there are no inches or centimeters or millimeters in web design - you specify widths and heights in pixels, percentages of em units.