YOUR OPENING LEAD...

This can be the toughest position, because often you only have this chance to beat the contract. 

Ask yourself are they in a suit or No Trumps?

P has bid or doubled a suit - lead that suit.

You have a strong suit of your own - lead that suit. 

You have a sequence of 3+ cards - lead that suit.
You have an interior sequence, lead the top card of the sequence.

You have a singleton and 2/3 trumps - lead it.

You have 4+ trumps - lead your longest outside suit.

It's No Trump - lead your longest suit (normally 4th highest), unless they've bid it.

Your hand is awful - look for P's suit. Lead your highest card in it.

They bid around everything - lead dummy's second suit.

Dangerous leads:
Away from a King; an unsupported Ace; Doubleton in a suit they have bid;
Away from an Ace against a suit contract; small singleton trump.
A lead from four to the Jack.