How do you make it so you put in a keycard and it outputs a redstone signal?

I am playing Tekkit with my friends and I can't figure out how to make a program where you enter a floppy disk in (a specific one) and it will output a redstone signal (to open a door say).

You would use the peripheral API to check the label of the disk, or the FS api to check a file on the disk, or some combination thereof, then use the redstone API to output your signal. There are probably even tutorials for this basic idea in the tutorials section.

And there are several examples of this concept in the Programs section. Take a look at other peoples' code, and you'll learn a lot from it!