Here's what would happen... most likely:
1) You will lose some weight
2) You will start seeing some changes in your body shape
3) You will think I'm eating the same stuff as before, I don't need to exercise any more
4) You regain all the weight because the exercise somehow burned off the excess calories you eaten before
However, if you eat healthy and throw out the junk food, you can loose weight without even exercising. Exercise promotes a overall better body condition.. you can think of it as what's on the other side of the door... diet is the key to open this door. Of course, you can break and enter this door by simply doing exercise, but pretty soon the diet police is going to come looking for you and when it does, you're going to feel it 10x.
Don't feel bad if you continue eating junk food along with heathy food. Just slowly take it off and you'll see how easy it is to maintain a healthy diet.