This is a complicated question but I'll simplify the answer.

If you are talking about getting 21 on the inital deal of two cards then that is a fairly easy calculation. You need to first find the chances of getting a 10 card and then find the chances of getting an 11 card.

10 card

out of 52 possible cards there are 4 each of: 10s, Js, Qs, Ks which means you have 16 ways of getting 10 out of 52 cards.

11 card

out of 52 possible cards there are only 4 kinds of Ace so 4 out of 52.

Then you multiply the probability of getting each card and you have your answer: (16/52) * (4/52) = 64/2704 = 2.37%

The answer can get more complicated when you start to think about how there is only 51 cards left after you take the first card etc.