[Mahou]

Define athletic - because it seems as though everyone has a different sort of standard for this.

I'm not going to beat a dead horse, because travis and reach already have put out some really good suggestions (minus buying the bowflex or treadmill).
If you really want to get started, I would try challenging yourself every day to do something different that keeps you somewhat active (i.e. going for more walks, roller skating, swimming, etc). The general idea behind this is to get you to ease your way into fitness. A lot of people tend to give up if they feel like they are being overwhelmed too fast from their exercises or from information that's being given to them.