How to think about leading.
The first consideration in opening leads is which suit to lead. After you decide which suit, then decide which card in the suit. The first rule in deciding which suit is LISTEN TO THE BIDDING The bidding will give you an idea of what the opponents have:
• How many points do the opponents have?
Determine roughly how many points the opponents have, how many you have and then, by process of elimination, how many partner has.The more points that partner has, the safer it is to lead from an honor. Leads from Qxxx and Jxxx are much better if partner has some points. They are dangerous if partner has very few points. Similarly, honor doubletons such as Qx, Jx, and even Ax are better if partner is known to have points, but are extremely dangerous if partner has few points. Doubleton honor leads are also better the weaker declarer is known to be, such as against an auction where declarer has opened with a preemptive bid.
• Do they have a long side suit that they can run?
If there is a long side suit, how is it breaking? If it appears to be breaking evenly, make an attacking lead. If you have it well under control, a trump lead is usually best. Why? Because, when dummy has a long suit, it also has a short suit. Since the long suit cannot be established, declarer will try to use dummy's trumps for ruffing. Each trump you lead is one less loser that declarer can trump.
• Are they stretching for their contract or is it comfortable?
If they are stretching, you should be passive, sit back and wait for your tricks rather than attacking, if they comfortable maybe you can make aggressive leads.
• Should I be looking for a ruff?
Lead a short suit, particularly with a trump holdings that look like xxx, Kxx, Ax (x), but not with QJx or J10xx where ruffing costs you a natural trump trick.
• Should I be looking to give partner a ruff?
This can work out well if you have a quick trump entry facing a partner who is marked with shortness in a side suit.
• Should I be leading a trump to cut down on ruffs in the dummy?
Declarer has bid two suits, winds up in the second suit, and you have length and strength in the first suit. This is a good time to lead a trump.