I would do a normal cardio routine, include some sprints, and then stretch for 30 minutes or so. Look into Matt Furey or Pavel. They have great stretching routines.
Being limber will help you not tap out too easily in certain situations but cardio is much more important when grappling, IMO. Strong abs will also help you do some armbars.
BJJ is alot of technique so don’t worry too much about preparing, just get there. You’re gonna love it!