Simply, if you can count an A as 11 points, and after adding other cards' scores, total score stays at 21 or less, you should do it. Otherwise count it as 1. It's hard to get more specific without revealing too many details =/

Click over Blackjack link in the kata description to read the wikipedia article. It's by choice.

["A", "J"]                      ==>  21 <- "A" here is chosen to be an 11