The cosmopolitan is a classic drink, albeit a rather girly looking one. Don't be fooled, it packs a decent punch, just make sure you're ready to be seen at a bar with a pink drink.
Mix the vodka, triple sec, lime juice, and cranberry juice in a shaker with ice, and put that shaker to use. Once it's cold, strain the drink into your martini glass. The cherry is optional, but fits perfectly with the color of the drink, so don't avoid it if you have one around - just put a maraschino cherry on a plastic tothpick and plunk it in there.