The standard leads from honour combinations are:
Ace from A-K (but king from A-K doubleton).
The higher of two touching honours.
Top of a doubleton.
Low (eg fourth highest) from other combinations.
1. Some players prefer to lead the king from A-K.
2. Against a suit contract, it is usual not to underlead an ace.
3. Against a no-trump contract, the lead of an honour usually guarantees the possession of at least three honour cards unless the lead is from a short suit. From a long suit headed by just two (touching) honours, the standard lead is a low card (eg fourth highest).