Haven't done a wall of text for a very long time, so here's one.
It's actually easy to explain. We use a base-10 system when counting, which means that one digit could have 10 values (0-9), this is called decimal.
Notice the first few letters of that word, you can also find it in "Decade" (10 years), "Decameter" (10 meters), "Decagon" (a polygon with 10 sides) and "Decane" (C₁₀H₂₂).

Hexadecimal is a base-16 system, each digit could have 16 possible values (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F). The prefix can also be found in "Hexadecagon" (a polygon with 16 sides) and "Hexadecane" (C₁₆H₃₄).

Here's a list of decimal numbers from 0 to 255 and their hexadecimal counterparts, it should make counting in hexadecimal clear: https://pastebin.com/raw/qcdh6WaS

When converting from RGB colours to hex, you convert R from decimal to hex, then G then B and add all of them together, for example, rgb(28, 16, 54), 28 is 1C in hex, 16 is 10 in hex and 54 is 36 in hex, if you add them together, you get #1C1036.

For conversion, you could use an online converter, a scientific calculator (used an emulator to record this) or even the good old windows calculator.

There's also binary, which has two possible values for each digit (0 and 1, the prefix can be found in "Bicycle" which has two wheels) and octal, which has 8 possible values for each digit (0-7, the prefix can be found in "Octopus" and "Octagon").