You don't really need motivation to get started, you just have to do it anyway, motivated or not. Start small, go walk for 20 minutes around your neighborhood. Repeat until it becomes a habit and part of your daily routine.

If you like animals and you are responsible enough to be a pet owner, getting a dog can help too.

Well, depression is a positive feedback loop where a negative mental state leads to actions that amplify that state. To break the loop, something has to change.

Eating works :)

I think it's more organisation than motivation, at least for me. I think most people if someone knocked on the door once a day and said 'hey were off cycling/walking/whatever for an hour, wanna come?' and you had the time available would go. The key is to make it fun and convenient, and then motivation creates itself.