1) If we roll a pair of dices we'll have the following combinations, the first number is for the first dice. The second for the second dice:
1,1 1,2 1,3 1,4 1,5 1,6
2,1 2,2 2,2 . (...)
3,1 3,2 3,3
(...)
(6,1) 6,6
Until we have a total of 36 outcomes.
2) So the probability of rolling two sixes is
P (6,6) =1/36
Because there is only one favorable result, over the total outcomes (36)