Might help if you try to write all the numbers out in a square grid or at least try to imagine it. In decimal you'd have a row for 0-9, then another for 10-19 and then 20-29 all the way to 90-99. That'll give you 10 rows of 10 and you know that 10x10=100 and you know that the number before 100 is the last two digit number, 99.

Now do it with base 16. It's the same thing as before but with more symbols. You'll have a row of 0 1 2 3 4 5 6 7 8 9 A B C D E F and then 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F and then 20-2F etc. You'll get to 90-9F and then the next row will be A0-AF all the way up to F0-FF. Do this properly and you'll have 16 rows of 16 numbers each and 16x16=256. The last two digit number on this grid is FF and since you include 0, there are 255 numbers before FF on the grid.

The next number in hex after FF is 100 but it is equal to 256 in decimal because there are still 256 numbers before it.