Before you start learning to count the cards in blackjack, you should remember that the basic blackjack strategy is the starting point for blackjack card counting. Don't even think of blackjack card counting before you know your basic blackjack strategy perfectly.
Another example: If you have a running count of 5 in the first hand after the shuffle in a single deck game, you can see that this is much more meaningful then if you have the same running count in the first hand after the shuffle in a 6 deck blackjack game.
As the remaining cards in the shoe become less and less, this factor decreases. Card counters do not want to depend on luck, so we want to adjust our strategy so that it takes this factor into consideration. So we use a count that takes into consideration how many decks that are left - a count per remaining deck is also called The True Count.
Another example; If your running count is 5 and there are about 2 decks left - the true count is 2.5.
So as you can see, figuring out the true count is not that hard to do. The hard part is figuring out how many decks there are left in the shoe. To do that, you have to start practicing by looking at the casinos tray with used cards - calibrate your eyes and estimate approximately how many decks that have been played and thereby how many decks there are remaining in the shoe.
The true count is the backbone of blackjack card counting - this is what tells you the appropriate bet size to make AND when to vary from playing according to basic blackjack strategy because of the remaining contents of the shoe.
Good luck beating the blackjack game and remember to practice, practice, then practice some more!