Generally speaking, a low card indicates interest in defending in that 
suit and a high card indicates that there probably is not any interest
in defending in that suit.  This is true in suit contracts as well as
in no trump contracts.  There is a difference, however.  

Let's look at suit contracts first.  Here are some basic rules to make
the hardest decision in bridge a little easier.

1.  We do not underlead aces against a suit contract unless we are
absolutely sure it is the correct thing to do.

2. If partner has bid a suit (or indicated a suit) during the bidding,
we will lead that suit unless we are absolutely sure that it is correct 
to not lead that suit.

2a.  If we do not lead partner's suit, he has every reason to believe that 
we do not have any cards in that suit.

3.  If we can make a lead that will allow us to retain the lead (an ace-
king combination), we will do that so as to be able to see the dummy and
to see the card that partner plays to this lead before doing anything else.
This includes leading partner's suit - so this would be an acceptable 
reason for not leading partner's suit at trick one.

4. The lead of a low card from a non-trump suit indicates that the leader
either has the unsupported king or queen of that suit, or, that the card
is a singleton.

5.  we try to lead from suit combinations.  Some combinations are better
than others.  KQ is good, KQJ is better.  KQT is acceptable.  T98 is normal.
876 is normal.

6.  Holding AKXX in a non-trump suit, it is preferable to lead the ace.  If
you lead the king from both AKXX and KQXX combinations, partner may not be
able to tell which you are leading from.  The lead of the ace should show 
the king and the lead of the king should show the queen, unless you hold
AK tight, in which case you lead the king and then lead the ace to trick 
two.  Partner will know you have a doubleton when this happens.

7.  If you choose to lead from a suit with three small cards (any xxx), 
lead the middle card and when you have the chance to play to that suit 
again, play the highest card remaining, denying a doubleton.

8.  If you choose to lead from a four card suit that is not headed by
the king or the queen, lead the second highest card (except for sequence
leads).  From J973, lead the nine.  If the suit were Q973, you would lead
the three.

If you follow these rules, your partner will be able to place many of the
cards in the suit that you have led from the information that you have 
provided, the cards in the dummy, and the cards in his hand.  As you learn
to follow rules such as these, you will be able to complete a picture of 
the honor cards that you cannot see, making your defense easier.

Here are some rules for leading against no trump contracts.

1.  Has partner bid (or indicated) a specific suit?  If so, lead that suit.
You would lead your own suit if it were absolutely correct to do so or if
you had no cards in partner's suit.  Otherwise, don't guess.  When you 
refuse to lead partner's suit and you are wrong, you have not only shown
a lack of respect for your partner, but for your partnership as well.

2.  If you have a four card major, try to lead that suit if it makes sense
from the bidding.  A low card indicates you have an honor (ace, king, or
queen) in the suit.  A high card would indicate otherwise.  

3.  The lead of an ace asks partner to play his highest card in the suit 
led.  The lead of a king asks partner to play a spot card that shows his
attitude for that suit.  The lead of the queen asks partner to play the
jack if he has it.  These are commonly accepted lead agreements.  You and
your partner can make any agreements you want to make, but you should be 
on the same page with regard to these situations.

4.  Over time I have found that I have the most trouble when my holding 
in the suit that I want to lead is KQXX.  Invariably, if I lead small, the 
JX appears in the dummy.  If I lead the king, the ace appears in the dummy,
either doubleton or singleton.  Use your best judgement - if I ever come up
with a rule that works, I will re-write this rule.

5.  If you have no entries back into your hand, it is often best to try
to lead the suit that you partner should be holding - if you lead from a 
long weak suit you may never be able to regain the lead to make use of 
any tricks that may have been established.

6.  Pay attention to the bidding and make an intelligent lead.  The opening
lead is a blind play but the opening leader doesn't have to be deaf.
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