take the old one out noting if it has a wiring harness from the auto wiring to mate to the radio harness ie male/female connectors, if so cut the harness from the old radio and splice to the new radio soldering or electrical tape connections following the color coded diagram that comes with the new radio. If the harness is not already there buy a new one for make/model of auto and install. Plug in harnesses to each other and install new radio after connecting the antennae.
A wiring diagram for a Honda Civic can be found in an after-market repair manual or in an official Honda EWD (Electrical Wiring Diagram) manual. The former can be found in most auto parts stores, and the latter is found at most Honda dealerships. Mechanical diagrams can also be found in after-market repair manuals.