There are a number of devices out there to tracking your movement, sleep and other things. But you may own such a device already: it’s your mobile phone. If you combine this with this: Moves – Activity Tracker for iPhone and Android, then that may be all you need to get started.
If you are going to get it, look at the comments here, first: Swissmiss | Moves App. As you can see, there are some limitations to it. My recommendation: try this app first. If you find it useful, consider getting a standalone tracker from someone like Fitbit or Nike.
Anything that helps get people moving, fitter, and healthier is a good thing.