Define what you want to establish as a new habit. Nonspecific things like "get in shape" are not actionable enough. Be specific with your goals. Something like "work out at the gym for 30 minutes and run for 30 minutes every day" is a much better habit to try and establish.
Set a goal of doing your new habit for just 30 days. Tell yourself that if you do not like it, that is fine, but whatever happens, commit to doing it for 30 days. This is less daunting than a new lifetime habit, and is reassuring in that if the new habit really is not a good fit, you will not hate yourself for quitting.
Use reminders on the fridge, in the car and at your work place to remind yourself of your new habit.
Tell your family and friends about your new habit, and ask them to help you stay the course. They will help you, and you will have the added incentive of not wanting to look bad to your friends and family by quitting your new habit before the 30 days are up.
Reward yourself for sticking to your new goal on a monthly, weekly or even daily basis. Just make sure the reward is not counterproductive to the goal. For example, junk food is not a good reward for sticking to a new exercise routine.