I'd suggest watching Scooby-Doo, Where Are You!, The New Scooby-Doo Movies, Scooby-Doo and Scrappy-Doo, The 13 Ghosts of Scooby-Doo, A Pup Named Scooby-Doo and What's New, Scooby-Doo? in order actually.
Don't bother with Shaggy & Scooby-Doo Get a Clue!, Scooby-Doo! Mystery Incorporated or Be Cool, Scooby-Doo! unless you want to insult your intelligence.