When your RCX is new (right!) or has been without power for longer than a few minutes, it will have lost its firmware, which you must upload again. In this state the RCX is not completely useless. It still has three basic built-in programs that you can use.

Program 1

When the program is started, Output ports A and C are turned on forward.
So if you attached two motors to these ports and linked them to wheels, you have a rover that drives forward.

Program 2

Sensor ports 1 and 3 are configured for use with touch sensors.
When the program is started, Output ports A and C are turned on forward.
When touch sensor 1 is activated, motor A stops. When touch sensor 3 is activated, motor C stops.
With this you could for example build a rover that turns when hitting an object.

Program 3

Sensor port 2 is configured for a light sensor. Keep two motors attached to output ports A and C.
After activating the program, the motors will turn only when the light sensor is close to a bright surface, for example a white paper. When above a dark surface (for example a black line), the motors will stop.
With this you could build a Rover that drives on a white surface until it reaches a black finish line or boundary.