The single most important moment in a deal of bridge is the opening lead. It is a unique card played without a sight of dummy. It is very important to listen to the bidding to guide us to make certain choices.
What was the bidding of the opponents?
Did partner bid?
How many tricks do you need to defeat the contract?
In a No Trump contract it is a race between you, the defender or the declarer. The best defense is therefore to lead a long suit in anticipation to set up that suit.
Exception: When we have no "High Card Points" ( HCP) try to lead your shortest suit to find partner with length who will have the remainder of the points. After all Xmas will come before you set up a topless suit!!