I have been doing sit-ups and I notice that I feel pain/sensation in other parts other then teh stomach. When I do situps, my legs feel tired. when I do sit-ups, my back hurts. Why is that? I think that also prevents me from doing more situps..I feel sensation in those parts more then I feel in the stomach! Am I dong it wroing. what is the right way to do sit-u[ps??

First you should feel no tension besides your abdominals. Your form is wrong obviously. I do not know how you do a sit up but I would never lift my spine off the floor. The best thing are crunches. Lie down and put your feet on the wall and make them like their 90 degrees. Kinda like this:

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Crunch to the sky so to speak lifting your shoulders only and squeezing your abs.

If your doing your situps right your stomache would feel sore and thats the sign that it is actually working make sure your flat on the gorund and your but is not in the air.

This is a common problem and it has to do with form (as Just2Built pointed out). Your legs feel sore because you are using your hip-flexors. Legs should remain relaxed and you only should crunch to the point where your shoulder blades leave the floor. As you approach burnout, this will become a largely psychological effort, but if you can't hold the proper form, stop the exercise. Its not worth stressing the lower back.