Go to a bike shop and talk with the folks there. They can fit you onto a bike that's right for you. A well-fit bike will go further than anything in keeping your motivation to ride. I kept a spreadsheet for every single ride I did (time, miles, avg, speed, etc.) and that helped me gauge my progress. It also kept my motivation sky high, as I saw the pounds flying off.
Good luck! You can do it!!! Everyone here is pulling for you!