Exercise bikes do help to burn off fat as does any form of exercise. Just do half an hour or so a day every day and providing you are sticking to reasonable diet you should lose weight. I don't think that there is any excercise that specifically reduces stomach fat although some will tone the stomach muscles.
Thats OK.
I bought a cheap bike about a years ago and its been really good .. but don't try to do too much or you get fed up and stop using it. Its better to do a little but do it every day.
Good luck!!!