get a mic, plug it into the "mic in" jack on your soundcard (adapter may be needed), fire up the program, create a new track, arm the track to record audio from the mic jack, hit record, and try not to choke up.

Usually a preamp, mixer, or high quality sound card is used, to reduce noise and improve quality. Always monitor, while singing, through headphones.

That should more or less be how it goes.

Saul