The majority of electronic projects, and certainly mechatronics ones, involve input devices, signal processing and output devices. I suggest you familiarise with various types of sensors (e.g for light, temperature, pressure or simply switch operation) and the drive requirements of various types of output devices (e.g. relays, lights, servos, actuators, motors, displays). As for the signal processing, that may be analogue or digital but increasingly is done with a MCU (microcontroller unit). So learning to program a MCU is a good idea. The Arduino is a good start, in view of its popularity and the available tutorials and open-source code for common projects.