What is a force?
A force is making someone pick a specific card or object that is chosen before hand when they think they are getting a free choice. There are many ways to force a card in magic and here are a couple:
Cross Cut Force: Place the card you want to force on the bottom of the deck. Have the audience member cut the deck by taking the upper part of the deck and placing it next to the lower half. Next have them take the lower half and place it on top of the other half but have them place it so that the two piles make a cross. Next have them pick up the new top half and look at the bottom card which is the force card and have them remember it.
10-20 Force: Place the card to be forced in the 10th position from the top of the deck. Have the audience member pick a number between 10 & 20, lets say their number is 16, count down to the 16th card in the deck. Next add the two digits in the selected number 1+6=7, now count back 7 cards and turn the 7th card over and it will be the forced card.
Another Force: The card to be forced is on the top of the deck. Ask an audience member to pick a number between 1 & 20. Count down to the number and flip over the card and say "now this card would have been your card but I am just showing you how this works". Flip the card back over and place the cards back on the deck. When you counted, the top card to be forced was placed on the bottom so all you have to do is count down to the same number and you should come upon the card you want forced.