Is the tv show going to be exatly like the books?

No.
Some things close to the start of the show were almost verbatim from the books, and while the overall season arc is the same they really started veering away from the book around episode 7,8 on minor details that will later have a much bigger impact in the shows direction. Season 2 aside from the "Fellowship" storyline, is extremely DIFFERENT than the second book so far.

Most of the supporting characters (and Bill) are much different than their book versions, especially Tara and Jessica (who dosen't even exist in the books).

No not really, or at least that is what I can tell. I just finished the first book. I started into this great series about two weeks ago. I bought the first season of the show and loved it. I think when out three days ago and bought the first 8 books. After reading the book I don't see any way they can keep to the book 100%. It is the closest I have seen in a long time to keep to the book. They have added a few things and changed the importance of a few characters, but over all the basic concept of the first book was kept in the first season.