Basically, word to what everyone else here has said. Watch the show. Watch it from the beginning. Do not skip Season One.
There is a very important reason for this. Babylon 5, more than almost any other show ever made, is structured as a novel for television. You wouldn't pick up a novel and start reading at Chapter 7, would you? Of course not. You start at the beginning.
Now, Babylon 5 is not perfect. It definitely has its weak points, and Season One is the weakest of the lot. Episodes like "Infection" will probably have you wondering why you're wasting your time with the show and wondering what we were all smoking when we told you it was so great. But Season One lays a lot of crucial groundwork. There are plot threads that begin in the very first episode that play out through the end of the show. There are things that happen later in the show that, if you watch from the beginning, will absolutely floor you; if you start with Season Two, they'll still be interesting, but they won't have nearly the same impact.
Now, that said, there is one small exception to that. In my opinion, "The Gathering" isn't very good, and should NOT be a newbie's first impression of B5. If you've committed to watching the whole show, or at least giving it a while before making a judgment, then absolutely, start with "The Gathering." But if not, I'd say start with the first episode of Season One and go back and watch "The Gathering" about 1/2-3/4 of the way through Season One. It's not ideal, but it'll work.
And don't watch "In the Beginning" first, for reasons already stated in this thread. Watch it either after the end of Season Four, or the end of the show.
As for the rest of the movies, they're pretty mediocre IMO. And while I loved B5, I thought Crusade sucked. There were only two or three episodes I'd even rate as halfway decent, and the rest ranged from poor to utterly abysmal (I don't understand how "Visitors from Down the Street" ever even got made. Why did no one stage an intervention for JMS?)