Good news! The keyboard is one of the simpler instruments to record. Any keyboard worth its salt is equipped with line outputs. The line outputs plug directly into your mixer or recording device. Better yet, you won't have to eat up one of your valuable microphone channels, because the output level is sufficient for recording (that's what line level means). You can set the level on the keyboard simply by adjusting the overall output level.
Recording in Stereo
Keyboards, especially modern synthesizers, utilize a stereo signal path internally. That means most of the patches are designed for listening in two-channel stereo—left and right signals. If you output only one of the signals to your recorder (perhaps to save inputs), you might not get a full sound. This is due to the way the keyboard pans the sound from side to side. To get the best sound, use two tracks for a keyboard. The exception is if you're trying to record an older analog-style keyboard that you know has a mono output; then you can get away with one track. By programming your synth's internal parameters, it's also possible to “sum” the output to just one channel, to combine the two signals into one. The term “sum” means to combine many signals. A mixer is a great example of a device that sums many signals into one stereo pair.
For monitoring, you have many options. If you're laying down solo keyboard tracks, you can just plug a headphone into the keyboard. If you play with an amplifier, you can plug into that as well. If you're working with prerecorded tracks, you can monitor from the recorder with either headphones or monitor speakers.
MIDI or Audio?
Keyboards communicate with computers via MIDI. One of the benefits of using MIDI is that it's not audio, meaning you've got a lot more ability to edit the sound. With its simple code of commands to the keyboard, MIDI can be highly edited on a computer screen through all the major DAW programs. On the other had, if you record the keyboard signal as audio through an audio track, there's little you can do to change aspects of the performance. You would have to go back through the piece, decide what you want to change, and rerecord the audio. If you keep the performance as MIDI, you can tweak the part endlessly until you feel it is final.
Many keyboards contain built-in sequencers, which are recorders for MIDI. While the built-in sequencers allow you to record and make music with the keyboard's built-in sounds, the screens are small and editing can be cumbersome. This is why computer-editing programs are so popular with keyboard players the world over—big screens and mice make for powerful editing.
At some point, however, you need to record the keyboard's actual physical sound. By itself, MIDI and MIDI sequencers don't capture any “audio” that you can print to a CD. So, you have to plug the audio outputs from the keyboard into a recorder. Recording the audio from the keyboard allows you to make the final mix and bounce a final product to an audio CD.