The show is rather long and complex (but great!), so there's a lot to watch. Here is everything you should watch, in this order:
1. 2003 Miniseries (3 hour special before Season 1)
2. Season 1
3. Season 2 (this season box set should have six discs)
4. Webisodes: The Resistance (a series of ten 2-3 minute clips that can be found on Youtube)
5. Season 3
6. Webisodes: Razor Flashbacks (I think these can be found on Youtube, but I know that all 7 parts feature on the Razor movie DVD)
7. Razor (TV movie. It can be found on DVD, and Razor is also included (on the first disc) in the season 4.0 box set)
8. Season 4.0 (ten episodes)
9. Webisodes: The Face Of The Enemy (a series of ten 2-3 minute clips that can also be found on Youtube)
10. Season 4.5 (the final half of the last season. I think there is ten or twelve more episodes. The eleventh episode of Season Four airs on January 16th. The season finale could be up to four hours in length.)
11. The Plan (Another TV movie, approximately two hours in length)
Another television series, called "Caprica" (a spin-off of Battlestar Galactica) is to be released. It is a prequel to BSG, set fifty years before the events of Battlestar Galactica.
Sorry for my long answer, there's just so much to watch! I hope you like the show.