Well, you will have to find something that works for you. What motivates one person, won't work for another. Here are a random selection of tips:

1. Reward yourself. After I go for my walk then I can... (insert favourite treat/activity)
2. Start with incidental exercise. Every time you think "I need to get more exercise", do 10 squats. Grab your DS and do a bit of dancing.
3. Say to yourself that you will walk for 10 minutes and if you aren't enjoying it after 10 minutes then you can stop. (Most often people will enjoy it and keep going, but the pressure of not having to do a long session helps them start out).
4. Eat a meal outside. Great for getting daylight, get out of the house, improve your mood. That might help you kick start going outside to exercise too.

I often say to people that exercise for depression is like chemotherapy for cancer. Yes, it's not going to be pleasant and you won't want to do it, but you HAVE to in order to get better. Its your medicine.