The hardest part is the internal change. Once you decide, on your own, that you really want to change, then you can do it. That change can be for anything.
you need to evaluate what you consume. Everythign that goes into your body. There needs to be a reason for it. To do this, you will need to have a basic understanding of what makes your body work. And how what you put in will affect your body's functions.
I suggest you go to the library and get the book "The Schwarzbein Principle". She walks you through the basic biochemistry of your body. Not everything will apply to you. You identified the major components of your issue (sedimentary lifestyle, too much intake, not enough burning). But the concepts WILL apply.
Take things slow. You need to be careful about how fast you lose weight. And when you are not losing mass (according to the scale), you need to know how to identify body composition changes. Example: I am close to 180. I will never get below that. And honestly, I will probably go higher again. But my body fat will continue to decline. The muscle and bone will continue to grow in size and density. Thus increasing my overall mass. (Which sucks for climbing)
Do you have health insurance? Talk with them. They may have programs to repay gym membership (I have found the classes help keep me motivated), or even personal trainers. You may want a PT as you get into better shape. Actually, talk with the HR people @ work. They can help you with this. And should be very happy to do so!