If you've got a Spotify account, you now have access to about 40 DARK SHADOWS audio dramas. The available titles date back to the earliest entries in the range, including the four-part "Kingdom of the Dead," all the way up to more recent tales like the David Selby/Donna McKechnie reunion "The Darkest Shadow." (Strangely, Spotify is offering just the second volume of the acclaimed "Bloodlust" serial, but the prologue and first episode of that storyline are available for free from Big Finish HERE.)
If you're new to the audios, the best advice I can offer is to find a character you like and dive right in. Over the years, the producers have created a loose continuity between some of the stories, but there is no prerequisite study involved. If you already like Quentin Collins, for example, you'll have no trouble following any of his stories in whatever order you happen to discover them. How these episodes interact with each other is mostly a bonus.
Below is a sample from "The Crimson Pearl" from Spotify. It's one of my favorites in the line, and was a testing ground of sorts for the kind of extended tales Big Finish would later tell.