How many seasons you need to catch up on: Three so far, and hopefully one million more in the future.
Why it's important to start from the beginning: Season 1 essentially sets up the cutting catalyst for why a group of four families — The Starks, Baratheons, Lannisters, and Targaryens — go to war. (Get it? Cutting catalyst? Because of Ned and his... Oh, never mind.)
Even if you're a big fan of the book series, you need to watch Game of Thrones. That's because the show doesn't stick to the book at all times, it veers in very upsetting and unexpected directions. So, if for nothing else, you should watch these so you can bitterly lament how the writers have butchered George R. R. Martin's work.