What materials form the structural elements of the Boeing 737?
Most of the Boeing 737's structure is made in aluminium. Composites are used for smaller parts like the ailerons, the wing-to-body fairings and the tail cone.
It looks like all of the control surfaces are composites.

Medium capacity airliner series. Aluminium alloy wing structure, upper skin and tail plane, with graphite composite elevator, rudder and ailerons. Glass or carbon fibre-reinforced plastic nosecone, fairings and fin tip. Single or split-scimitar double winglets.

more composite components:
The nosecone, wing and fuselage fairings, fin tips, the fairings of the flap actuators and other non-stressed components are constructed from glass and carbon fibre reinforced plastics (GFRP and CFRP). The rear of the engine nacelles are constructed of graphite, Kevlar and glass fibre composites.
In February 2000 an advanced carbon graphite winglet developed by Boeing became available as an option on the 737-800.