Plz tell me how to open locked doors for example the one in dragon stone where we're supposed to find the blacksmith's son or the door in the Iron Mines???

You bash them. Hold shift and bash them, if you see damage numbers you know you can destroy them. Also press Z and make your companions attack it too.

Alternatively, learn the Pagan magic Shatter and cast them on locked doors. It will open most locked doors. Doors which cannot be opened by this method will require key(s) or levers, buttons or panels to open.

You can also use the heretical spell Decay. This one is giving me better results than Shatter now.