I love to hate King Joffrey.
You have no idea.
So far, about as faithful as a television adaptation could be. All the main characters' plots are pretty much spot on.
There are some additions to deal with certain things that are implied in the books but not necessarily spelled out - for instance the scene with Loras shaving Renly did not happen in the book, and it obviously makes them gay characters. This is implied multiple times in the books, but not so blatantly. (many people were shocked at it and the implications had to be pointed out to them) I've heard season 2 also has some planned additions, but as you've not gotten through the first three books I won't spoil it for you.
On the other hand some things are left out to a degree. Bran's dreams about the three-eyed crow go into much more detail in the books. Also, the dire wolves play a bigger role in the first book than they do the first season of the show. This is kind of inevitable though - you'll almost always get more detail in text.
Some things were made different for television as well. Dany in the book is 13, but she's made older in the show for reasons related to television. Bran is also a couple years younger in the books. The Mountain that Rides is also larger in the book - kind of hard to find an actor eight feet tall and scary enough I guess.