Well, basicly hex is just easy as dec. Idea in hex is that it's based on 16 numbers. (Dec is based to 10)
So, it has 16 numbers which are:

CODE

0 (0 in dec) 1 (1 in dec) 2 (2 in dec) 3 (3 in dec) 4 (4 in dec) 5 (5 in dec) 6 (6 in dec) 7 (7 in dec) 8 (8 in dec) 9 (9 in dec) A (10 in dec) B (11 in dec) C (12 in dec) D (13 in dec) E (14 in dec) F (15 in dec)

In dec-system we have numbers from 0 to 9.

If we count 9 + 1, we jump to "second tenth", which is like 10
In hex same operation is F + 1, and result: 10

So hex-system just has 16 numbers instead of 10.

As I before posted, there's a mathematic-way to change hex to dec. Hope it opens to you

It's perhaps easier to think with oct-system.
Oct-system is based to 8 numbers (from 0 to 7).

So 7 + 1 would be 10.

So it's all about kind of "rollpoint" where next "tenth" begins.

Here's some links which could also help:
http://www.danbbs.dk/~erikoest/hex.htm
http://en.wikipedia.org/wiki/Hexadecimal
http://www.stcsc.edu/OCL/hex_to_decimal_conversion.htm
http://www.the-eggman.com/seminars/about_hex.html

I hope this helps a bit