The probability of getting tails is 1/2 assuming we have a fair coin (both sides are equally likely).
The probability of getting two tails in a row is (1/2)*(1/2) = 1/4 since each coin flip is independent. You could flip two different coins at the same time, or flip a single coin twice, and it's the same basic idea.
I'm not sure what you mean when you wrote "19 of two tails".
