Create your workout routine based on your fitness/athletic goals and your time frame. For instance, you may want to lose 50 lbs. in 6 months for a wedding and reveal/build some muscle in the process. Perhaps you want to pack on 5 pounds of muscle and get shredded, or run a marathon. Whatever it is, define it clearly. Write it down.
Next, depending on your level of fitness and nutrition knowledge, start learning or hire an expert (personal trainer, coach, nutritionist, whatever works for your goal.)
If you don't want to hire someone, you could find a good resource on the internet, but find some good books as well, the internet can be a bottomless pit of sketchy advice. If you have little to no knowledge, what your are looking for is:
-Sensible advice on workout frequency for your age (how many times a week?)
-Workout progressions with your experience level in mind. (i.e. How many sets/reps/intervals/miles/minutes, etc., and when to increase weight/mileage and so on.)
-Easy to follow nutrition advice that doesn't promote anything.
Whatever you do, start slowly. Do less than you think you can at first and always, always, always warm up. Most importantly, start a workout routine and never stop.