You can start right away, but it's better to have played the first game. But the two games actually play very differently so not all players like both (plenty do, though, like me). The first game is also longer so if you're just playing it because you want to get to the second game you might get impatient.
My personal recommendation is to play them in order, as I think the first game is really interesting too, but I always tend to play things in order because I'm picky. Others I'm sure would have no problem jumping right into the second game.
The Witcher 2 has some references to the first game, but mostly it's just that you'll know many of the characters already. They're introduced again, of course, but it's better when Geralt meets his old friends and they're your old friends too since you met them in the first game. Also the situation Geralt finds himself in at the beginning of the second game makes more sense if you've finished the first game (but it is, of course, recapped quickly in the second game too).
Ultimately it's up to you though... if you think the first game looks interesting, definitely go for it. I think it's great, despite some shortcomings and roughness. If you're not as interested in it, then go ahead and jump right into The Witcher 2.
