Take a deck of cards and shuffle the bottom card to the top. To do this, simply hold the deck face up in your left hand (so you can see the bottom card), and with your right hand take all the cards except the bottom card and drop them, a few at a time, back into your left hand. The bottom card will naturally fall into the crook of your fingers in your left hand, and, when the deck is turned face down, it will now be the top card. If you execute this quickly enough, it will look like a perfectly natural shuffle, and as far as your spectator is concerned, the trick hasn't even started yet. However, you now know the top card.
Hold the deck in the standard dealing position. Place your thumb on the top-left corner and run it down from the top card to the bottom card. This is called riffling, hence the name of the force. Nonchalantly ask your spectator to say "stop" any time she wants.
Pull down with your thumb to create a break in the deck when she says stop, drawing her attention to the card underneath your thumb. This is the card she thinks she will pick. As a courtesy, ask her if she's happy to take that card or whether she would prefer you to continue riffling to select another. It makes no difference either way, but it makes the selection seem fairer.
Gently grasp all the cards above the break with the thumb and index finger of your right hand. Pull them away, while maintaining a slight pressure on the top card with the middle, ring and pinkie fingers of your left hand. Your right hand will remove all the cards above the break apart from the top card, which will silently snap down directly on top of the card she thinks she has selected. If you perform this quickly enough, the effect will be completely invisible. She will take that card, believing it to be the one you stopped at. At this point, you can take things in any direction you want. You can pretend to read her mind and then tell her what her card is, or get her to put it back in the deck for you to "find."
The riffle force is a perfectly good card trick for any beginner. You can then learn the drop force and the classic force. In the meantime, practice this one until you're confident enough to perform it.