• "Dark Disciple" finishes Ventress's story. (Which would've been finished in the show if it hadn't been cancelled.)

• The "Son of Dathomir" comics explains what happens to Maul after his last appearance in the show. (Same as Ventress, it would have been shown in the show if it hadn't been cancelled.)

• The "A New Dawn" novel serves as a prequel to Rebels.

• The comic series "The Last Padawan" also serves as a prequel to Rebels, but if has a few flash forward scenes, that seem to be taking place in the middle of season 1.
  No real spoilers for season 1 though, so you can still start with the comic if you want.

• There's also a few short videos that serve as prequels to Rebels, you can find them on youtube.
  They're not very important though.

Other than that you can just move straight onto Rebels.

After you start Rebels there's still a bit of extra material:

• There's a short book series called "Servants of the Empire", it takes place during season one.
  It's not absolutely required, but it does tie up a plotline from season 1 that is otherwise unresolved, and it's a decent read.

• There's also a novel called "Thrawn", which features the backstory of a major character in season 3.
  I'd recommend reading this novel before watching season 3, it's completely optional though.