You need to have forks that accept disc brake calipers and wheels with hubs to accept the disc. I have 2 sets of disc brakes and calipers still in the box because i didn't know that when i bought them.
Ended up getting a drum brake and having a bike shop make the wheel. They go on most forks (front). Plus I don't know if it's doable to have a rear disc brake on a bike with a rear sprocket attached.