If you can, rent or borrow Disk 4 of the first season and watch the unaired pilot. Alas, it's the only episode that's not being streamed on Netflix.
Otherwise, start with season 1, episode 6, "Man on the Street" and watch the rest, except for the second episode of the second season, "Instinct", which is both unnecessary to the main story and boring.
Dollhouse is not Whedon's best work, but if you like what he does, it's worth watching the good bits version.
Emma noticed something I think is true: ultimately, Dollhouse is Topher's story.