Start small and build up a routine. Try to use the stationary bike (or walk outside) for 15-20 minutes a day, either in the morning or in the late afternoon. Intensity doesn't matter so much as getting your schedule and routine adjusted does. Do this every single day for a week- set reminders, give yourself an incentive for completing it, etc.

Next week, increase the time you spend. 5 minutes, 10 minutes, maybe even more if you feel like your body can handle it. The following week, increase either duration or intensity. Bike more, bike at a higher speed, walk more, shift the walk into a brisk walk or a light jog. I think once you get into the habit of exercising then you can start to follow a specific plan and begin focusing on muscle groups to strengthen!