I would just like to add to this, that for the duration that SG-1 and Atlantis were running simultaneously, the new episodes themselves ran in parallel. SG-1 season 8 and Atlantis season 1 each released new episodes on the same night (two part season openers not withstanding), back-to-back. And while rare, they will later on make direct references to things happening episode-by-episode in the other.
This is the one I use. I like how it switch's back and forth between sg1 and Atlantis. My first time watching I didn't start Atlantis till I finished sg1 and it was a bit confusing going back in time.
Best place to start is the original Stargate movie as it sets up the universe and the characters for SG1.
Then, one you hit season 8 of SG1, Atlantis starts and there is some timeline cross over between the two. It's not essential to switch between the two to understand what's happening, they can still be watched pretty independently of one another, but switching between the two to watch it in all in chronological order can still add a bit of extra clarity to the earth's and atlantis' interactions.
Typically, the only place I could find an actual link to one of the episode lists I've seen in the past was through reddit, so I've just copied and pasted the link to the list, rather than the reddit url. The user was deleted otherwise I'd give them credit too.
https://docs.google.com/spreadsheets/d/1b5T6bk5bS9LDaIVXR55-agOd_FYgTN0TZcpZDfvnMg4/pub?output=html
Start with the movie but realize it's not nearly as good as SG1's quality later on. Then watch SG1 and if you keep watching by the time Atlantis comes about you'll already know if you like the show enough to continue. I don't recommend anything after Atlantis and the SG1 movies but some people like Universe so it might be worth checking out.