Planning your play 

Decide which suit offers the greatest number of potential tricks and lead it first. In most cases, this will be the longest suit between Dummy and Declarer. While you have control be prepared to give the opponents their rightful tricks in your long suit if you can set up tricks for yourself by doing so. Your decision should usually be controlled by the holdings you have in the suits contained between yours and dummy's hand, also affecting your choice is whether you are in a Suit or a No Trump contract. 

No Trump Contracts

Start playing the suit which helps you to win as many tricks as possible, this is usually the suit in which you and dummy hold the largest number of cards. So, count the combined cards in each suit and generally the suit with the most cards is the one you should start with.
If you have a suit with certain losers but you have a long holding, you should try to lose your losers; forcing out the opponents' high cards in that suit (trick promotion) before you lose control (with your stoppers) in your side suits. 

Suit Contracts

In suit contracts unless you need to ruff   losers in other suits first in order to develop additional winning tricks you should begin by drawing the trump suit . This is a suggestion not a hard and fast rule. If  your quick tricks count shows that you have enough for the contract, then you should draw the trump suit first. This is the best way for avoiding a ruff (trump by the opposition).