Its important to note from the outset, that you can't count cards playing online. Why? Because even if you know how to count cards, each new deal in blackjack (or any card game for that matter) at online casinos, is from a full shuffled deck/shoe. Otherwise online casinos would be basically giving money away to smart players...and we know this would never happen. Nevertheless, if you are also a keen terrestrial gambler, or are just interested in how the pros beat the casinos, then here's a brief summary of how to count.
By counting cards, a player can ascertain whether, at a given point during a game, he/she has an edge over the house. The key to knowing whether or not the player has an edge over the house, is having an idea of whether the remaining cards in the deck/shoe are predominantly high, or predominantly low, as a high card rich deck swings the odds of winning in favor of the player.
Reasons for this are as follows:
An abundance of high cards increases the likelihood of blackjack being dealt to both the dealer...but blackjack to the player pays 3 to 2.
The dealer has to hit on a stiff hand (12 to 16), while the player doesn't, and when the deck is high card rich, a stiff hand hit is more likely to bust.
The probability or winning from double down and split plays also increases when the deck is high card rich..
So if you know the remaining deck is high card rich, you know that you have an edge over the house - and to do this you need to track the cards already dealt. But don't worry, because this doesn't necessitate memorizing every card that has been dealt, but rather tracking the cards by assigning a tag to the cards and keeping a running count of the tags. Tags are as follows:
low cards (2, 3, 4, 5, 6) have the tag +1;
high cards (10,s, pictures and Aces) have a tag of -1;
the rest (7, 8, 9) have a tag of 0
The count before the first deal is 0. As cards are dealt, you simply add the tags of every card dealt so that at any time during the game you have a running count that may be positive, negative or 0. Now, if during the game, your count is positive, this means more low cards have been dealt than high cards, and as a result, the remaining shoe is high card rich. At this point, your odds of winning are greater than at when the game first started...and if the count is significantly positive well into the shoe, then you may even have a significant edge over the house.
You can practice you counting at home by dealing a deck to yourself and counting as you go. If your count is 0 when when the whole pack has been dealt, then you got it right. Your counting really needs to be automatic before you can actually count successfully in a real game situation, because remember that you still have to think about how to play your hand, well as deal with general casino distractions.
Now you know how to count, you need to know how to bet based on the count. In simple terms, you play basic strategy, and up the ante when the count is positive, and lower the ante or sit out when the count is negative.