Kitty84 asked:

"how guitar chords relate to keyboard (?)"

That one's easy. The notes in "guitar chords" are exactly the same note in "piano chords". Chords is chords.


"Example: If I'm trying to write a song and the melody/vocal is going to hit a "C" note, does that mean I need a C chord? When do I need to switch to a different chord?"

Usually, when the melody dwells on a note, the chord has the same note in it. The main chords that include C are:

C (major = CEG, minor = CEbG)

F (major = FAC, minor = FAbC)

Am (minor = ACE)

Ab (major = AbCEb)

If you are used to reading "sheet music" for playing piano, you already know how to play chords, even though they aren't marked as such, in the notation. Most commercial sheet music comes with guitar chord charts above the treble clef. When the chord chart says "C", odds are high that the notes in the piano part are made up of (mostly) the notes of a C chord, in that same part of the tune.

To get a feel for how to decide when to change chords in your songs, learn a lot of songs, and pay attention to how the chords are arranged in repeating patterns. You will start to get ideas for "new" patterns, which are called "chord progressions". If you actually come up with something that has never been done before, it will be near miraculous. Never mind, just enjoy discovering stuff that's "new to you".

HTH
Emmit Sycamore