If the service panel does not have room for new circuit breakers and you cannot use tandem breakers, a subpanel may be the answer. Before installing one consult, with an inspector to make sure you do not overload your overall system.
A subpanel has separate bus bars for neutral and ground wires and typically has no main breaker. It may not be labeled "subpanel" but instead be labeled "lugs only." It may be a different brand than the main panel.
Have the inspector approve the subpanel, the feeder cable, and the feeder breaker.
Shut off the main breaker in the service panel before you begin.
About 4 hours to install a subpanel with several new circuits, not including running cable for the new circuits
Screwdriver, hammer, voltage tester, flashlight, strippers, lineman's pliers, long-nose pliers
Stripping sheathing and wires, connecting wires to terminals
Run cables for the new circuits to the subpanel location. In the main service panel, make room for the double-pole feeder breaker.
Subpanel, mounting screws, approved feeder cable, staples or cable clamps, approved feeder breaker, breakers for the new circuits