Let me start out by saying that there are many ways to go here, but one of the world's best players, Bob Hamman, is happy to play "Standard." If it is good enough for him. ...
published w/ permission of Bill ButtleSpot-card leads
In "Standard" (which varies in different parts of the world), from holdings of 4 cards or longer, the 4th best card is led. So, from K9842, lead the 4. This is the same against suits and notrump. With only 3 cards, such as K63, lead low (the 3). With any doubleton, lead high. With a singleton...no, don't dignify the question by selecting which card you would choose. What about with 3 low cards (no honor)? There is no "standard." There are advantages and disadvantages to all three choices; just choose one and agree with your partner.
What about leading partner's suit? In the old days, it was correct to lead highest. These days, you should (generally) lead the card you would normally lead (so, from Qxx, lead low).
Leading HonorsFrom Ace-King suits, it used to be normal to lead the king. This was confusing since the king was also led from king-queen. Now, most players lead ace from ace-king. (Only if it is a doubleton ace-king should you lead the king). From other sequences, lead the top card. Against notrump, there are some special situations. One is to use the Queen lead from KQ109 as looking for an unblock of the jack. Ace and King leads against notrump are special. The most popular method is to lead the Ace for Attitude (like from AKxx or AKx or AKJx), and the King for Count or Unblock (like from AKJ10 or AKJ10x). In other words, the King is the "BIG CARD." It is easy to remember this due to the alliteration. Whatever method you choose, agree with your partner on what an ace or king lead shows against notrump.
Other
Many experts lead their lowest card from 3- 5- or 7-card holdings. From 4- or 6-card suits they lead their 3rd best. This helps their partner read the distribution. I don't recommend this method for most players (not worth the effort to change).
There are other "non-standard" opening lead agreements such as:
> Leading Attitude against notrump (the lower the card you lead, the more you like the suit).
> Rusinow Leads: You lead the second of touching honors (so from KQ you lead the Q, from QJ the Jack).
> 10 or 9 shows 0 or 2 higher cards, Jack denies a higher card. This is very good against weak players as you give your partner a useful piece of information. Meanwhile, a poor declarer won't appreciate the inferences available. On the contrary, against good players, this is a horrible method. It gives way too much of an advantage to a decent declarer.
I'll let you in on a Bergen-Cohen secret: We used to play this 10/9 0 or 2 method against weak players, but standard against good players. We had 2 different convention cards depending on who our match that day was against. Thank goodness, our opponents never knew our criteria!
Next month: Defensive Carding/signals