It is impossible to tell you what to lead. It all depends on what you hold and what you can learn about the distribution of the cards from the play. Remember that you are playing with a partner, so it is important to watch what he plays. He will often signal what he wants you to do. For example, if you hold, say AKxxx of a suit, it is normally safe to lead the Ace. If partner wants you to continue the suit he will play a high card under your Ace, whereas if he wants you to switch, he will play a low one. If he plays the Jack under your Ace, you continue with the King. When he plays a low one on the King he is telling you that he has no more of that suit left, so you can lead a third low card, knowing that he can ruff (trump) it.