45 isn't very old. In fact I just saw a story about a 73 year old grandmother who looks more fit and lean than I do, so I need to start lifting my game a little.
Start by just going for a walk every day. Cut out junk food as much as you can. Treat yourself once in a while but make it once a week, not once a day! Remember you don't have to try to train like an elite athlete from day one, you just have to get active, make sensible food choices and the weight WILL start to come off.