The number of cards you deal depends on the number of players. Typically, if you have two players, you deal seven cards to each; deal five if you have more than two players. Put the rest of the deck facedown in the middle of the table -- this is your fish pond, where players go fishing when they need to pick up a card.
Go Fish tests the honesty of your players -- it's easy to cheat -- so set some ground rules before you play. Tell players that they have to follow the rules when they steal cards or have them stolen. If they don't, they'll miss a turn or will automatically lose.