Just because you've decided to begin a running regimen doesn't mean you have to be ready for a marathon your first day out. In fact, if you've never run before or are getting back into the sport after some time away, pushing yourself too hard can lead to injury, which is guaranteed to bruise your motivation.
Stay injury-free and eager to run by taking it easy at first. Make it a goal to run to the end of the block and back. Or try setting a time limit. Run for 2 minutes your first day, and add an extra minute a day until you're up to about 30 minutes. When all you need to do is jog at an easy pace for 2 minutes on your first day, how can you say no? If running for 2 minutes straight is too much, do as much as you can, and walk the rest -- the important thing is to keep moving.
Once you're jogging 30 minutes per day, consider upping your pace so you can cover more ground in the same period of time. You'll be surprised at how easy it is to compete with your personal best and how good it'll feel every time you reach a new goal.