It's a cousin of the Margarita. As such, go for a 3:2:1 ratio of vodka, triple sec, and fresh lime juice; i.e. 1-1/2 oz vodka, 1 oz triple sec, 1/2 oz lime. As far as vodka, the main difference between the various brands is marketing. I stock Smirnoff around here and am perfectly happy with it. Just don't get the cheapie stuff in the plastic bottle and you'll be fine. Triple sec... I go with ol' DeKuyper. Cointreau is the king of triple secs, but I find that the difference in quality between DeKuyper and Cointreau to be not worth the steep premium. Marie Brizard is an excellent brand as well, but I don't stock it just because hardly anyone around here carries it.
And use fresh squeezed lime juice, from real limes you've just squeezed. You have no excuse not to at home.