All pretty simple stuff as others have mentioned, just wanted to offer my own spin I guess (more opinions can't hurt right). As everyone else has mentioned, diet is more important than exercise in an either or scenario, if you're eating too many calories you won't lose weight – simple as that. However exercise itself is super important too (once you've thought about keeping diet under control). First figure out what how much of what you're eating, figure out what you need for maintenance and eat below that. Strictly speaking, even if it's McDonald's you can still lose weight (see here, interesting stuff). Ideally you want to start mixing in better foods if that's an issue but your goal is to be making an entire lifestyle change, not some plan to lose fifty pounds that you'll gain back in a year. After you've got diet under control figure out the first steps you can take to do some cardio everyday and what's holding you back. Weather too cold over winter? Pickup a gym membership. Don't like the gym or feel you're not getting value there? Grab a fold up treadmill you can throw under your bed. Don't like running? Invest in an elliptical. I'm kinda running off on a tangent here but break down whatever is in your way, find what works for you but most importantly GET STARTED AND KEEP CONSISTENT!
Good luck.
