First, you'll want to be comfortable but sexy. Choose something that's not going to have you tugging at your hemline all night, nor is so tight that you're constantly worried you might burst a seam. And, if you're looking for a longer-term relationship, stay away from overly revealing clothing that will give the wrong impression.
Now, assuming you still have choices left in your wardrobe, let's move on to colour. The first consideration in colour, obviously, is that it suits you. You need to be looking your absolute best. We all have colours that we know flatter us, and we tend to stay with them. There are, however, shades of almost any colour that will suit your skin tone, and some colours give off better "first date" signals than others.
Black is a slimming colour that is chosen by many for that very reason. It's also a great evening choice, as it's easy to glam up and, as a rather blank canvas, it allows your face to be the focus. Be careful, however, as black can send signals of authority, arrogance and drama. You can soften this effect by accessorising with playful splashes of bright hues.