HEART CONVENTION
When you have the first lead and your partner has doubled a "no-trump" make you are expected to lead your highest heart. As there are very few hands where it is advisable to double "no-trump" on general strength, it is necessary for the leader to know what suit to lead when his partner has doubled.
When the leader holds an A K or an A K Q suit he should first lead the K of that suit and then his highest heart.
WEAK-SUIT CONVENTION
In England and in some parts of this country the leader tries to guess his partner's suit by leading the one in which he himself is weakest. While this convention affords many more opportunities of doubling, it is not nearly so safe as the heart convention. There is about an even chance that the weak suit led will put the dealer or the dummy hand in the lead.
THE NON-DEALER'S PLAY AGAINST A DECLARED TRUMP
The principles of play adopted against a trump and against a no-trump declaration are entirely different; and it is for this reason that Bridge is confusing to the beginner.
The important principles that govern the play against a trump declaration are:
To hold the lead in order to see the dummy hand.
To make high cards before they can be trumped.