Then entire half life series is easily in my top 3 best single player games, its such a great story and the way its done (that is, without cutscenes) gets you so in-depth with game, you become completely emerged in the game.
That said, you dont HAVE to play the first one, HL1, but some things will make a bit more sense, you'll get a few more jokes, and in general everything will be a bit better.
But if you start with HL2 you'll be MORE than good to go.
I wana go play HL2 now. lol.