The probabilities in Blackjack are rather interesting to figure out. Basically, you will be trying to see how many cards are left in the deck that can help you with the hand verses the amount of cards in the deck that will cause you to bust. The ones that will help you will not necessarily get you right to twenty-one, but will get you closer. You will have to recalculate the odds after every card that you take since they are constantly moving as long as the makeup of the deck is changing.
To show how this is done, think about this example: You are playing at the table with one other player. That player has two Kings — this is unlikely, but it will make for a simple example. You also have a King and Two. The House has two cards, but you can only see one and it is the last King. Now you need to figure out if you should Hit on your card total of twelve or if you should stay where you are. You need to know what is in the deck. First, note that anything from a Nine down to an Ace will help you — the Nine giving you twenty-one exactly. Since there are four suits, that means that there are thirty-six cards that can help you in a standard deck. That leaves sixteen cards that will cause you to go over. Now you can subtract what is on the board. Your Two has to come out of the cards that will help, reducing the total to thirty-five. The four Kings have to come out of the other side, bring the total to twelve. That means that the odds are thirty-five against twelve in your favor. You certainly want to hit. You might get one of those twelve and lose, but the odds say you are likely to get something that will help.
It should be noted that online Blackjack uses a new deck for every hand. Counting the cards over the course of the entire game, then, is useless. You are always playing with a standard deck.