Fitness is basically split into cardiovascular fitness, muscular strength and flexibility.
You need - something to get your heart rate up, for 30 or so minutes, a few times a week at least. That can be whatever gets your blood pumping at your own fitness, whether that's walking, couch to 5k etc.
You also need a bit of resistance work - again this can be anything from less sweaty pilates, or maybe bodyweight circuits of things like squats, lunges, crunches, push ups etc (joe wicks sort of stuff, popsugar) which have the benefit of also being cardiovascular, and you will gradually want to add weights to these. You want something that will hit most of your muscles each week in at least 1 session.
Mobility or stretching work - yoga with adriene is great and will also give you a bit of strength benefit. Just make sure to stretch after any workout!
If you have any joint issues, I would recommend swimming, which is cardiovascular, low impact and has some strength and toning benefit.
If you're easily bored, something more fun such as dance workouts or a sport will help you stick to a regime. If you hate your workout you won't do it.
If you don't want to think about it, most of the fitness DVDs like jillian Michaels have good mixes of cardio and strength with some progression.