I think:

Your first stop should be a full physical exam. Tell your doctor of your plans in detail.

Next, look at your diet. This doesn't need to be a night-and-day change here. You can try to limit certain things (e.g. Sat Fat) and replace with other things (e.g. fruits/vegetables). Don't go for a diet change to just to lose weight, your emphasis should be what changes you want to make for the rest of your life.

For stone trail riding (like a restored railroad bed), I recommend a Hybrid bicycle. For paved surfaces, a road bike. The road bike may not be the best choice for a first bike, but get some miles under you belt and some fitness. Knowing that you can reward yourself with a new bike is a good motivator to continue.

Start with trail riding (It's a little safer). Get used to wearing a helmet. Don't walk up hills (pull over and rest instead). Having a triple (three chainrings up front) should give you the gearing you need.

Keep it up and by this time next year, those hills won't seem as steep.