Hello Danomatic,
What software or hardware are you using?
You would need a DAW to record and mix your music, some common ones are Logic, Protools, Ableton, FL studio, and Cubase.   If you are using a Mac then you should have garageband already installed.
How to record though is different depending on if you're using hardware or software.   You can hook a keyboard up to the computer via USB to control the software with.  You can select it by going to preferences / audio & midi settings / active midi inputs 
Regards,
Rebecca