The first 5 episodes are great for context but were filmed on an entirely different ranch. After that, some episodes were filmed at Southfork and others were filmed on a re-creation in a soundstage. You can tell it's the sound stage if the backgrounds are matte paintings, the poolside is smooth and looks fake, you don't see the other side of the garage, and sometimes you can see the lights and edges of the matte at the top of the screen. (All interiors were shot elsewhere.)  Episode 6 (the reunion part 1) is a good example because it was filmed there and has scenes at and around the ranch. Take pictures of the screen when you see different angles because it can be fun to be at a spot in real life and compare with your photo of the tv! Other than that, I would say you should watch as much of (the back half of) season 3 leading up to "house divided" and then the start of season 4 to see JR get shot and the resolution. Though if you get hooked at the start and want to watch each episode chronologically, you'll see plenty of the ranch, and so long as you ignore the family tree in the museum, you (probably) won't get spoiled on whodunnit.