There are two decisions to make when making the opening lead: which suit and which card from within that suit

SUIT CHOICE

Partner's Suit: if partner has bid nearly always correct to lead this suit.
Longest and strongest: try not to lead a suit bid by opponents unless your holding is strong.
Unbid Suit: listen to the bidding as it may indicate the best lead.
If you hold a very weak hand with no outside entries it may be best to try and find partner's long suit.

CARD CHOICE

Partner's Suit:Top of a doubleton:  9 5. K 6.  Small from an honour:  Q 6 2.  J 7 6 4.
MUD:  8 5 3.   Top of a sequence (2 or more)  J 10 5.  Q J 6.  10 9 4.
REMEMBER a low card promises an honour (unless singleton!)
Top of a Sequence or Near Sequence:  A K Q 6.  K Q J 7.  Q J 10 7 5.  J 10 8 6.  K Q 10 5 3.

Interior Sequence: Lead 2nd highest:  A J 10 7.  A Q J 6 3.  K J 10 5.  Q 10 9 8.

Fourth highest from 1 or 2 Honours:  K 8 5 3.  A J 7 5 2.  K Q 9 6.  A K 7 4 3.
REMEMBER a low card promises at least one honour.

Second highest from Weak Suit:  9 7 4 2.  10 8 5 3 (10 does not count as an honour when leading low but does count as part of a sequence)  8 6 3.

Weak Hand:  Try to find partner's suit:  ♠ J 8 7 3 2   ♥ 9 8 6   ♦ 7 4 2  ♣ 8 5.
After 1NT - 3NT my choice would be ♥ 8.  A spade lead needs too much help from partner and you have no entry.

General Points

If the bidding goes 1NT - 3NT and we have a choice of suits it is better to lead a major as declarer will not usually have 5 and dummy has not used stayman.

If stayman is used and no fit found after a major suit response it is better to lead a minor:

1NT - 2C - 2S - 3NT

Declarer has 4 spades and dummy has 4 hearts as otherwise he would not have used stayman.
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RULE OF 11

When partner leads 4th highest deduct the value of the card from 11 and this will tell you
how many higher cards are held by you, dummy and declarer.
                       9 8 3
    K 10 7 6 2                 Q 4
                      A J 3
6 from 11 leaves 5 between the other 3 hands: you can see 2 in dummy, 1 in your hand so
declarer is known to have 2 cards higher than the 6.

                      J 9 4 2
      A Q 10 7                  K 8 3
                         6 5
7 from 11 is 4.  We have 2 and dummy has 2 so declarer has no card higher than 7.
If dummy plays low, play 8 which wins.  Then play K and then 3,  If dummy plays J or 9, with with K and return 8.
If we play K at trick 1. dummy will make a trick with J.

                      Q 6 4
      J 9 7 3                       K 10 5
                      A 8 2
3 from 11 is 8.  3 in dummy, we have 3 so declarer has 2 cards higher than 3.
If dummy plays low then we paly 10.  We know that partner has either the J or A as he led LOW card.
If declarer plays A then when partner gets in he can lead J through the Q 6 and we make 3 tricks.
If declarer plays low then we must switch to another suit and when partner regains the lead can again lead J through Q 6.

                        K J 
      9 8 4 2                      A 10 7
                     Q 6 5 3
8 from 11 is 3.  Dummy has 2 and we have 2.  This makes 4 so that we know partner has led second highest from a weak suit.
Declarer has always got 2 tricks in the suit and if we have a suit of our own to establish we need to win and lead that strainght away.

SIGNALS

When partner leads and we are playing a card that will not win the trick, we play High to encourage or Low to discourage.
                       K 6 4
      Q J 10 5                    A 7 3
                       9 8 2

If partner has lead an honour we know he has a 3 card or better sequence.
If we hold an honour ourselves we should play it immediately to avoid blocking a suit.
                       7 4 2
    K Q J 5 3                  A 9
                      10 8 6

If there is a long suit in dummy and no outside entry,  then we need to give partner the count in case he
has the Ace and can hold up so that declarer can only make 1 or 2 tricks. 
High card shows an even number,  low card an odd number.

                    K Q J 7 3
      A 6 4                         9 5
                      10 8 2
When K is played we play the 9 to start Hi Lo and partner knows decalrer has 3 cards and can hold up
until the third round.  Declarer only makes 2 tricks in the suit.

                    K Q J 7 3
      A 6 4                         9 5 2
                      10 8
This time we play 2 and partner knows we have 3 cards and delarer only 2.
He can take his Ace on the second round and declarer ony makes 1 trick.

When partner leads a low card we play the lower of touching honours.
                       9 6 5
      K 7 4 2                      Q J 3
                      A 10 8