The key to choosing the best suit to lead is to listen to the auction.  The standard guidelines are:

1.  Lead partner’s suit
2.  Lead the longest suit, however avoid the opponents’ suits, especially if it is their major suits.
3.  With a weak hand and weak suit, try to find partner’s suit.

The basic idea when defending against a notrump contract is to take advantage of the opening lead by going after the partnership suit that has the potential for developing the most tricks. This is typically the longest combined suit held by the partnership. Even if declarer has one or more stoppers in the suit, once they have been driven out, the defenders can take any remaining winners in the suit once they gain the lead. There’s no trump suit to help declarer.

•  When you have touching honors in partner's suit lead the top of the sequence.  
•  With 3 or more cards in partner's suit lead low when you have an honor and middle without an honor.
•  With a doubleton lead high.
•  If you hold a singleton then you may lead your own suit if you have a fairly strong honor sequence in that suit.  

If partner hasn’t bid, we typically pick our longest suit as the one with the most potential for developing tricks. With little else to guide us, leading “fourth from longest and strongest” is one of the oldest maxims in the game... and it works remarkably well! Still, there are exceptions.
•  We try to avoid suits bid by the opponents, especially major suits.  
•  Without useful entries to your long suit, guess which suit might be helpful to partner (opponent's unbid major, especially suits with honors)

With a choice of two long suits to lead, we tend to pick the stronger suit. The more strength we hold in the suit, the less help we need from partner to develop tricks.  

If the opponents reach 3NT and have not used Stayman or Jacoby Transfer, there is a major suit bias. The opponents are playing NT because they don’t have a major suit fit.  However, they may be playing NT even if they have a minor suit fit because NT scores higher and requires fewer tricks to make game.  Therefore, they tend to have the minor suits and partner tends to have the major suits.  So, you should tend to lead majors unless there is a compelling reason to lead the minor. When your hand is very weak, lead your short major because that is partner’s long suit and she has the points to set her suit up and cash it if you help her by leading it.

This is the easy part. Having chosen the suit, there are specific guidelines about which card to lead.

We were all taught that you lead the top of three touching honors or the top of a broken sequence (2 touching skip one like QJ9).  Computer simulations show that it is equally important to lead the top of two touching honors to prevent the opponents from winning a cheap trick.  So, lead the A from AKxx, the K from KQxx, the Q from QJxx, the J from JTxx, and the T from T9xx.

You always lead the A from a long suit headed by AK except for when the AK are your only entries.  In that situation, you must lead fourth best in order to make sure that you still have your AK as entries for partner to return the suit and cash it.

This is nothing new but it can’t hurt to reiterate that you should lead the Q from AQJxx, the J from AJTx and KJTx, and the T from AT9x, KT9x, and QT9x. Remember that a sequence starts with T or higher!  From T9xx lead the T.  From 98xx lead fourth or the 8 depending on whether you play 4th or second best leads.