Start with little things then find something really fun to do.
Adding a bit of exercise can be as simple as parking at the end of the lot instead of closer to the store. Take the stairs instead of an elevator whenever you can. Walk to the store instead of driving.
There are all sorts of food/exercise journals that can help. I use FitDay (http://www.fitday.com) because it not only logs my food but also my exercise, body measurements, weight and has a diary for anything else I want to put in. I particularly like the calendar feature as it lets me see my calories/fat/carb/protein amounts for a whole month without having to try and figure it out myself. Studies show that if you are tracking what you eat, you will be more successful.
Find a favorite activity to do. One that requires a lot of movement. Wii Fit is one that I have found really helpful. During the better weather months I love to take my dog for a run with my bike or for long walks. (We are up to an hour long walk!) I also like to de-stress by walking on my treadmill with my favorite tunes. Mix things up so you don't get bored. :)
Once you get started back up you will start to feel better, which will help with your energy levels and you will want to exercise more.