You cannot spot train, it is a workout myth. That is, you cannot do an exercise for a part of the body to lose fat in that area. Situps won't flatten that stomach nor will squats make your legs thinner, those only workout the muscles, which will not show through unless you burn that layer of fat.
The key is a combonation of cardio, strength training and a good clean diet. Cardio will burn fat overall, strength training (weights, resistence bands...) will build muscle, which boosts metabolism. For every pound of muscle you put on, you add an extra 50 calories an hour your body burns that your muscles require to function. Compare this to a measily 3 calories an hour a pound of fat burns! And ask anyone that is serious about working out and they will tell you that 80% of weigh loss starts in the kitchen! Eat a clean, healthy balanced diet. 5-6 small meals a day spread out 2-3 hours after each other to keep your blood sugar and metabolism stable.
So to lose those big thighs and legs, I suggest combining a cardio routine (at least 4 times a week) and a weight routine (at least 3 times a week, done every other day)
There are a lot of great books and websites you can google for weight routines. Or join a gym (the dreaded g word) and talk to a personal trainer there.
For the people asking about losing weight just on the bottom, your body is the only judge of where you will lose fat. Its a general rule that you will lose fat first in the last place you gained it. So don't be discouraged if your always-been-fat-thighs aren't catching up with the rest of your body, you have to have patience.
Low impact cardio routines I would recommend would be yoga or pilates, pilates is especially good because there are a lot of laying exercises.
As for everyone, most results are visible after 4-5 weeks, about a month.