Card Forcing is one of the most fundamental things you'll want to learn in magic.
There are many different methods of card forcing, and if you really become a serious card-trick magician, you may even wish to invent some of your own.
To get you started, here's a simple to learn, and easy method to force a spectator to choose the card you want him to pick.
You start with the card you want the spectator to pick on the top of the deck. To make this seem even more legitimate you might shuffle the cards quickly one or two times, making certain that the top card always remains there on the top.
Begin by asking your spectator what day of the month they were born on. If the number is larger than 15, ask them to divide the number by two, and if the answer is not a whole number, then add one to the number. If the number is less than 3 or 4, ask them to double or triple it. (It doesn't matter which -- you just want a number more than 1 or 2.)
It really doesn't matter what number they come up with, all you're doing at this stage is creating the impression that they are deciding what will be happening.
When they've come up with a number, ask:
"Do you think you can remember that number for a few minutes?"
(If they say "No", ask them to pick a number which is close to it that they could remember.)
When they have finally decided on a number (let's suppose
they say "seven"), you say,
"Your number is 7, right?, OK. In a moment, I'm going to pass you these cards and I'd like you to count off the number of cards which you just said, like this: One, Two, Three, Four, Five, Six, Seven."
As you count, you deal one card, face-down on the table, for each number you say, so that the person sees what you want them to do. Then you pick up that small pile of cards and place them back on the top of the deck in one pile.
Your spectator will think you're demonstrating what you want them to do, but what you've really done is move your chosen card down to the seventh place. When they pick up the deck and do what you just showed them to do, they will un-do what you've just done and the card you want them to choose will be at the top of the pile they deal.
Now when they count out the number of cards you can ask them to memorize the last card they deal off the deck, without showing it to you ... or to do whatever your trick requires, but you know, without looking at it exactly what the card is, because that is the one which you originally placed on the top of the deck.
This is just one of many ways to force a card on the spectator. The whole idea of a force, is to get them to choose the card you want them to. The nice thing about this force is that you don't need to learn and practice any difficult moves ... the trick lies in misdirecting the person's attention about what you're doing.