Depending on what stepper motor you have (or want to buy), there are different control ways. Usually stepper motors have datasheet with electric instructions about how to activate them. There could be also the need to develop or download some pieces of code (maybe python, for example) to map motor control on your rpi pins. I've tried some 28BYJ-48 stepper motor, which are very cheap hardware and very easy to wire and configure. So, with a very affordable price you can test simple robotics projects. I tested them with my RPI Zero W and published on my blog the related guide. Main things that you need to be aware:
- power needs for stepper motor: your RPI can supply up to 3.3V or 5V but low amperage, so you could need to assemble a battery pack to give external power to stepper motor
- GPIO map of your RPI: if the stepper motor is controlled directly by GPIO you must use the correct map related to your RPI version
- requirements foy your project: if you have speed or strength needs, you must consider it in choosing the correct motor