Set a realistic goal that you'll be able to reach. Post it somewhere where you can see it daily. You can set a final goal and some smaller monthly goals to reach to keep yourself motivated. Treat yourself when you reach your goals. Stick a dollar in a jar each time you exercise and when you reach your goal go buy yourself something with the money in the jar.
Start counting those calories. Sign up with My Fitness Pal. http://www.myfitnesspal.com/ It's free and you can even get an App for your smartphone. You can log your calories and your exercise wherever you are. It calculates everything for you. They have a huge database full of calorie amounts for the food you eat. They'll even give you a calorie goal when you sign up for your account. You can use the online community for fitness ??? and have friends to keep you accountable. Ever little thing you put into your mouth counts. Even those last few bites of mac and cheese that's left in pot. I know how easy it is to rack up those calories and not even know it. You can basically eat whatever you want as long as you account for the calories. But if you have a piece of cake for dinner, chances are you wont be full and you'll eat something else later and then you'll be over your calorie goal for the day. My best advise in this area is... if you know that your going to want dessert later with the family hop on the treadmill at some point in the day for 20-30min. It's all about balance. Depriving yourself will only lead to a private party with you and a box of cookies behind the pantry door.
Get moving. Find something you like to do.Walking, Running, Spinning, Yoga, or Zumba, anything that gets you up and moving. If you don't enjoy it you probably wont stick with it. So try to find something you like or at least don't mind doing. I like to run. If I don't find time for it I get cranky and if I take too many days off in between runs I pay for it on the next run. That keeps me motivated. I also set goals for myself to keep me moving. I have a half marathon scheduled for this May and a 15k in July. I have to register and pay up front for these and they're not cheap. There is no refund if I slacked off on training and I can't run.
These steps need to become a way of life. There's no going back once you reach your goal or you'll only have to go through it all over again.