It's usually best to just start with the book called Foundation. Prelude is of course a prequel in that it's about the young Hari Seldon and how he first got to Trantor, the Imperial city planet--which George Lucas ripped off for the Star Wars movies with Coruscant. Starting in Foundation's Edge (1982) Asimov began tying his Foundation Series together with his Robot novels (the Galactic Empire novels where always part of Foundation universe) and his time travel novel, the End of Eternity. R. Daneel Olivaw , Elijah Bailey's robot detective partner from the Robot novels, is revealed to have a major role in the Foundation universe. Aside from Olivaw and Seldon, the best character in the series is the mutant empath known as the Mule, who is both a mass-murdering tyrant AND a tragic figure.
Apple TV is making a 10 episode series based on the original "trilogy" with Asimov's daughter as a consultant. If you're interested, the BBC in England did a radio play series based on the first three books back in the early 70's. Here's a link if you ever want to listen to it. https://archive.org/details/IsaacAsimov-TheFoundationTrilogy