Have heard that I shouldn't smoke because I could get dry sockets. Will I definitely get them if I do?

Dry Sockets. No, but you greatly increase the chances, especially on lower teeth. It is always best to follow directions.
Should not. Post oral surgery instructions demand that after extractions that you should not drink through a straw and also you should not smoke. Sucking in from both of these actions can create a healing problem, including the heat from the cigarette. Follow instructions!