I am currently building my first Arduino project : a car that you can control from your computer. I have my Arduino Mega that outputs 3.3 or 5V and my stepper that receives 12-25V, how can I make my Arduino digitally write something to that stepper?
Use an H-bridge motor driver. They can use arduino signals to control higher voltages without damaging the arduino. Check out the L298N from gearbest. It's cheap but will probably do the trick if you're not using too much current.
It can supply about 1A through each gate at 5-30V.
this video should help explain it. (I haven't watched the video but it seems like it got good feedback).