Against suit contracts, standard leads are:
Use the bidding, especially your partner's, to determine what suit to lead. Generally do not underlead aces or kings without a good reason.
Against no trump contracts, standard leades are:
Again, use the bidding to select the appropriate suit. Unless your partner has mentioned a suit, lead from your own strongest suit (though be careful, and realize you may finesse yourself). With several honors lead as above, otherwise follow the ``fourth from your longest and strongest'' rule. Underleading aces in a four (or more) card suit is fine, but be wary of underleading kings. If your partner leads fourth best, remember the Rule of Eleven: subtract the spots of the led card from 11. The result is the number of cards higher than the lead which your partner does not have. This information may help you decide what to play.