TOWER: The air traffic service in charge of control of local traffic, take-offs and landings at an airport; indicated aerodrome control service.

GROUND: The aeronautical station providing surface movement control of aircraft on the manoeuvring area.

If you are under control of ground you only switch to another frequency when directed, or are granted permission. They'll tell you "Contact tower on XXX.XXX", for example. This change of frequency will normally occur prior to the line up and take-off clearance.