"I don't have time."
No one does. You have to make time. Schedule activity into your daily routine -- and make sure that it's the last thing to drop off your schedule when you get busy, not the first thing.
Bring a friend along. Get a portable music player. Switch activities, routes, locations, or times for exercise. Get creative and keep moving.
"I'm too tired."
Once you get into a routine, you'll find that exercise actually gives you more energy. It helps you sleep better too!
"I'm too fat to exercise."
No one is too fat (or old, or clumsy) for physical activity. After all, our bodies are made to move -- and regular activity helps you lose excess weight so you can move more easily. Think of it as a 2-way street. Being active helps you lose weight, and weight loss helps you be more active. The key is to find activities appropriate for you at your current weight -- it doesn't have to be running or cycling.
"I can't stay motivated."
Nothing motivates like success. Set small, realistic goals for yourself. For example, start with 10-minute walks every day this week, 15-minute walks the next, and so on, until you reach your long-term goal. And be sure to celebrate each milestone!