An easy trick that you can learn quickly.
1. Shuffle a standard deck of cards. Have your participant pick a card to memorize...and then have them put it back on TOP of the deck.
2. Bring the deck behind your back and flip the top card over so the card face is facing up. Bring the deck back in front of you facing the crowd with the bottom card facing them -- and the original top card that they selected facing you.
3. Ask them if this is the card that they selected. They will say no.
4. Put the deck once more behind your back and flip the top card back over face down and put it anywhere back into the deck.
5. Bring the cards back out in front of you, allow the participant to reorganize the cards. Now that you know their card you can search through the deck in any fashion you want and easily pick out their selected card.
Notes: As with all the card tricks on this site, you should have a deck of cards in your hand and go through the motions of performing the trick so that you can comprehend it readily.