Yup start with SG-1. Then you can either watch all of SG-1 then the movie then on to Atlantis, and finally Universe. Or Around season 7 I think, you can start intertwining Atlantis and SG-1 episodes if you really want. Since they are both running at the same time and both make brief mentions to each other at times heh
I actually watched all of Atlantis before I saw all of SG-1, because I'd seen a lot of episodes of SG-1 on tv but none of Atlantis, but now I'm up to season 10 (final season) of SG-1, so good!
When I finish this I'll watch universe, I've never actually seen an episode of universe.