Getting fit....?

rather than sit ups you might try laying on your back and raising your legs ..... helps to avoid hurting your back...

when I feel like exercising I usually make myself sit down till the feeling passes

"Fit" can mean a lot of different things depending on your goals. If you want to be sport-specific fit. . .like for Cross Country. . .then run (after you take a couple weeks off)

if you want to be fit to do daily activities in general one of the best lifts you can do is a squat but it is hard to do correctly so don't do it unless someone teaches you how. Then include something aerobic and also and something for balance.

If you just want to look "fit" well that depends on what that means to you as to how you would go about it but in general if you just want to loose weight and get a little muscle tone then whatever you do just do it consistantly and with proper form and that's all you really need to know/do.

Preferably pick something you like and you will more likely stick with it.