Wonderful question... I to have spinal stenosis, so I know what you are dealing with.  I believe the best place to start is by talking to your Dr. about setting up a eval with a good Physical Therapist.  It's also important to find a therapist that has actual training in osteoporosis, because I have found that many PT's have a minimal amount of education in this area.  Ask your Dr which types of exercises would be good to strengthen your core, and back extensor muscles since it's these muscles that will eventually protect your spine from further injury and flatten you tummy.  The American Physical Therapist Assoc (APTA) has a good web site where you can look up clinics in your area.  When you go to the web site look for PT's with specialized training in osteo and spinal injuries that would include a comprehensive routine/tx for both problems. 

You might want to ask your Dr if there is a specific exercise or tx that will help release the pressure in your spinal canal (stenosis) as well which will help with the pain. Proper body mechanics will be of real importance to you, so you can learn how to move correctly throughout the day during normal activities that you'll face.  You probably already know what I'm talking about if it's difficult to pick things up off the floor, reach into low drawers/cupboards etc.  You'll need to learn alternate ways of doing these things without causing further problems by bending from the waist-not good.  There are many aids that can be purchased that will help with these daily obstacles.  There are so many things that can help to assist you that it would be impossible to list them all here, so ask your Dr and PT for a good catalog for daily assisted aids.  If they can't think of any just let me know I have many good ones I could list.

A good place to start would be to get a good sturdy reacher, so you don't have to bend from the waist to reach for things that are out of arms reach.  Reachers can be used for a lot of things like getting laundry of out the washer or dryer, or picking up light weight things of the floor, I even used mine to pick cherries off of a tree :)  But don't do things like that until you get the docs ok.

If you don't have insurance that will cover Physical therapy, find out how much it would cost to get an evaluation that would consist of several visits with a PT, and then have them give you a printed routine with pictures that could be followed at home.  Actual PT is best, but since some don't have insurance to cover this, you could use this approach as a substitute, but you'll have to be diligent in following your Dr and PT's recommendations, and don't hesitate to go back for assistance if you have any trouble at home. If you have to do this, make sure that during the initial visits the PT monitors your exercise for technique so you aren't doing an exercise wrong, and make sure that the routine is ordered by your Dr for your two particular problems (stenosis and osteo).  I think you'll find that sit ups and stomach crunches are no nos, and they don't work very well any way for building up your core muscles, but the good news is there are plenty of other exercises that are safe and effective.

Good luck with all this, and come back if you need further assistance.