how to use the sp-3 pedal? do I press on it using the whole foot on it or I just press on the outer side? I bought one but I don't know how to use it?
Assuming you mean the Casio SP-3: you use it like you'd use a sustain pedal on a real piano. Keep your heel on the floor and press the pedal with the front part of your foot. Those electronic pedals tend to have weaker springs than a real piano pedal, and don't normally give you quite the feel of the real thing.
It works just like a doorbell push. Press it, sustain, let go, sound stops. Not quite as easy to use as one which looks more like a proper piano pedal, but does the job. Heel on floor, press with toes.
If your piano has a 'half-pedal' facility, then really you need a pedal that terminates in a TRS jack.