The correct way to begin using a card counting method is when there is a shuffling of the decks. You can't begin to count cards in the middle of the deck because you have no idea what the previous cards were and your count will not be accurate.
Also, practice counting at home, either with a deck of cards that you deal out yourself or at a free online game. You need to learn how to count quickly and accurately. This can only be done by practicing. Only rely on card counting when you are comfortable with it. Don't rush it.
The most popular card counting system is the Hi/Lo blackjack card counting system. I think it’s also the easiest to learn, compatible with the most popular blackjack variations and is therefore the first blackjack card counting system you should learn.