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
Whoever put together that list was very in depth, so if you'd prefer to follow a more generic chronology, this one from gateworld doe the job https://www.gateworld.net/news/2009/05/stargate-recommended-viewing-order/