There are many versions of the Margarita but to my mind simplest is best - take four ingredients -
2 oz of Tequila such as Patron Silver Tequila,
1 oz fresh lime juice,
1/2 oz sugar or agave syrup and
1/2 oz orange liqueur such as Triple Sec or Cointreau.
Pour into a shaker over ice and shake until properly chilled then strain into a glass over ice, crushed or otherwise.
Salt is traditional but optional. I prefer it without. If you don't have sugar syrup or agave to hand just add extra Cointreau or Triple Sec. And please don't go out and buy a Margarita Mix - it is not at all the same as the real thing.
The Margarita cocktail goes way back and there are many trying to insist theirs was the first, however Jose Cuervo (think Tequila), was running ad campaigns for the Margarita in 1945, with the slogan, 'Margarita; it's more than a girl's name.' According to Cuervo the cocktail was invented in 1938 by a bartender in honour of Mexican showgirl Rita de la Rosa.
So it may all be a little fuzzy, but whatever the history this is one of the most popular and well known cocktails and it couldn't be easier to make.
If you're interested in iconic cocktails and their background have a read @liquor.com