Well you have already had a basic explanation of how to begin card counting, but there are a few more things that you need to know before you reach your card counting glory.
There are also some blackjack games that you will want to avoid with card counting. Online blackjack and blackjack tables with Continuous Shuffle Machines. The reasons you want to avoid those blackjack games are very similar.
I will start with CSMs. CSMs are devices in which the cards from one round are fed into the device and the entire deck reshuffled before the next hand is dealt. Hence the continuous part.
This causes none of the cards that have been played to be discarded, and without discards there is nothing to count. Card counting is a way of keeping a loose running tab on the cards that have been played and removed from play under the remaining deck hits its reshuffle point. But since CSMs keep cards from being removed from play there is no point in trying to count them.
As for online blackjack, it is powered by a Random Number Generator. An RNG functions similarly to a CSM, but not exactly.
When an online blackjack game is created, its RNG is programmed with every possible outcome, such as all possible starting hands and dealer up card combinations. When a player clicks Deal, the RNG pulls from all possible starting hands and dealer up card combinations. This has an effect like a CSM because no cards are ever ‘discarded,’ it is possible for any of them to show up in the next rounds.
Because cards are not effectively discarded in a blackjack game with CSMs or RNGs card counting becomes useless in those games. It is best to stick with a standard game of blackjack in a brick and mortar casino—one without a CSM.