Where to start.....that would depend on your health and goals. Many people here do water-only fasts for extended periods, sometimes even past 40 days. Personally I find that scary, and don't want to risk electrolyte imbalances and stones in my liver/gallblader.
I'm opting to do a juice fast like a few other. I'm relatively healthy and think this won't mess up any body functions. I've heard this also causes stones though, so I might add a spoonful of fats or oils every couple of days.
And then there's the raw diet route.....raw vegetables, fruits, nuts, fats. This gives you the same calroies basically as a juice fast (as long as you eat less protein/fats), but you also get the good stuff like the fiber. This is also the lowest risk to your body, in case you're worried.
Basically it comes down to your personal risk/benefit level. You can lose weight on all these options, but you have to decide between the risk and speed.
As for exercise, I don't really know about back problems to give opinions. Have you tried pilates? Some people find it useful in strengthening the back and it reduces pain. But tread cautiously since there are some high impact versions that will do the opposite. You can also try gentle rebounding, it's extremely good exercise.
We already have 6 members. You can sign up if you like...it's easier this way since one message goes out to everyone, keeping things simple. Plus I'm sure there are people more knowledgeable than me who can give you advice on stuff.
If you feel like giving up, don't despair! Just remember there's a whole bunch of people going through the same thing as you in respect to diet struggles.