The lead to the first trick, before the dummy appears. The opening lead is often the single most important play on a deal. Before making the opening lead, you should consider the bidding and your own hand. At duplicate, having selected your lead, you should place it face down on the table.
The best opening leads are those that are both safe (unlikely to concede a trick) and attacking (likely to set up tricks). A sequence such as K-Q-J-x is usually a good lead. A suit partner has bid is usually a good lead too. Against a no-trump contract, you usually lead from a long suit, aiming to set up long cards as well as high-card winners.
When, as often happens, you do not have an ideal holding in any suit, you will need to weigh up, from the auction and your hand, whether to prefer a passive lead or an active lead.
If you can see that suits are breaking badly for declarer and finesses are likely to fail, you are more likely to go passive. If the auction sounds confident and your hand holds few surprises for declarer, you are more likely to attack.
A trump can be a good lead if you have a good holding in declarer’s second suit. Leads from unsupported honours, especially aces, tend to be risky.