You should be able to pedal backwards without doing any harm, just dont do it when you are changing gear.
There are several potential causes of clicking, but locating the click is never easy, it could be the bottom bracket shell or bearings, the crank mounting system, the pedal bearings. Pedal bearings are the most likely, the easiest the gain access to, and the easiest to fix, so I usually start there.
If you are a mechanical newbie, then you are probably better off taking your bike to a decent local bike shop.