Probably the easiest ones would be... Basic Step DVD, then the Low Impact Step DVD. Then after you feel more confident maybe try Athletic Step or Party Rockin' Step 2. To make things easier you can lower/remove the risers, just be on the platform if needed, or practice the moves just on the floor at first.
In addition to ones already noted, I’d suggest going way back in time to Cathe’s Classics DVD and her Volume 1. Those DVDs are probably $12 to $15 and you get 3 Step Workouts on each. The programs are absolutely timeless, even though the workout clothes are Blasts from the past. I absolutely recommend these for getting into Cathe Step routines. She builds slower in these and queues flawlessly. Many of the moves she uses in her later programs are introduced in these. So much fun! Good luck!!