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.

Lead the A when you hold a side suit headed by AK.  The lead of the ace allows you to look at the dummy and get an attitude signal from partner.  If partner encourages, you can then continue.  However, if partner discourages, you can shift suits and still hold onto the K to capture declarer’s Q.

When you lead a side suit singleton, you can get an immediate ruff.  You can get a delayed ruff.  You might also make transportation tough for declarer because she can no longer play that suit without you ruffing.  You should almost always lead a side-suit singleton, even if it is a suit bid by the opponents.  The only time not to lead a side suit singleton is when you have no trumps or when you have natural trump tricks, such as KQJT.  Even singleton honors are a good lead.  Although, you should avoid a singleton K if you suspect that declarer has the A.  She may take a finesse into your singleton K.

When you are defending, and your partner has bid a suit, it is usually a good idea to lead that suit. You want to be able to establish and cash those high cards quickly before declarer has a chance to discard his losers in that suit. Leading touching honors or from three or four low cards in partner’s suit works out very well. From a two-card holding lead the top card.
Holding the Ace in partner’s suit makes for a difficult decision about leading their suit.  The modern style is to overcall very aggressively (or even open the bidding aggressively) so we certainly cannot count on partner to have the King of their suit whenever they bid it.  This means that leading the Ace may give up a trick and it may be best to save our Ace to capture one of the opponent’s honors in this suit.  Nothing is absolute, but without anything else fabulous to lead, leading the Ace in partner’s suit from Ax (doubleton) is much better than leading from Axx or Axxx.  Leading Ace doubleton has the added advantage of possibly getting us a ruff if partner has the King or if we can get partner the lead before trump are drawn.  Also, when we are shorter in the suit (like with Ax instead of Axx), then the opponents are more likely to be longer and have more losers.

You should lead the unbid suit unless you have a very dangerous holding such as AQxx or KJxx or your holding in one of the bid suits is exceptionally strong (usually a 4-card honor sequence or better).

Leading high from small doubletons tends to work out well.  Even though you get a rough in the suit very infrequently (certainly a lot less than people think), you do not give away a trick on opening lead.  In addition, you are usually leading partner’s suit, because, if the opponents have not bid the suit and you are short in the suit, it is almost certainly partner’s suit.

3-card or longer sequences are fantastic leads and are second only to side-suit singleton leads.  Leading the top of two touching honors is almost as good as leading from three or four little cards and is better than leading from Kxx or Qxx.

A popular saying is: “Don’t lead away from an Ace against a suit contract.”This is a good piece of advice. From suits like Axxx, AJxx, & AQxx, do not lead the A and do not lead low.  Avoid the suit entirely.  If forced to lead the suit, lead the A.

Note that this only applies against suit contracts.  Leading away from an Ace is common against no trump contracts.  Declarer  won’t later be able to ruff our Ace.

Computer simulations have shown that leading from a side suit such as Kxxx, KJxx, and KTxx ends up costing nearly as much as leading from a suit headed by the A.  So, avoid leading those suits.  

Many of us were taught that when in doubt we should lead a trump.  Computer simulations show that leading trumps is one of the worst leads, should be a last resort, and that you should almost never lead a singleton trump.  Declarer usually draws trumps right away.  So, all that you are doing when you lead a trump is helping declarer.  There is a big difference between making an opening lead against a dummy that has trump support as opposed to one that does not.  It is almost never right to lead a trump vs. a dummy that does not have support.

You should only lead a trump in a few instances:
1. When you have honors (bad holdings from which to lead) in all the other suits,
2. When you think declarer is going to trump in the hand with the short trumps or cross-ruff, and/or;
3. When you have strength in the opponents’ side suit(s).

-   Small Slams

Against small slams lead a side suit A if the opponents used blackwood without cue-bidding to check whether they are off the AK in that suit (partner may have the K).  Also lead a side-suit A if you have length (usually 5 or more) if you think partner might have a singleton.  At matchpoints it is often right to cash a side-suit A in order to avoid letting the opponents make seven.  You should avoid leading an Ace if you think declarer has the K, such as she bid the suit or made a control bid in the auction.  Without an A, lead the safest lead possible, a 3-card or longer sequence or from two or more small cards.  If you have a choice between passive leads, lead an unbid suit.

-  Grand Slams

Make the most passive lead possible.  Lead from a 3-card honor sequence or two, three, or more low cards.  When choosing between passive leads, choose an unbid suit.  As a last resort, you may lead from two or more trumps.  Do not lead a singleton trump.