They say it takes 21 times of doing something before it becomes a habit. Whatever you decide to do, the key is to make the commitment and do it consistently.
To start, find a time that best fits your schedule -- the time you're most likely to work out and when your energy level is highest. If you're a morning person, work out before your workday starts. For night owls, exercise can be a way to unwind in the evening.
Picking the right environment is another way to make sure you stay on course. Do you get energy from being around a lot of people? Join a workout class, group or a team sport. If you prefer to work out alone, find a quiet, safe place to do so. If the weather outside is going to stop you from exercising, have a backup plan and place to do your activity, such as walking in the mall instead of the park. A workout buddy or group is also a good idea, so there's someone to hold you accountable.