How to write songs on guitar or piano?

Q. I have no clue how to write a song. I mean do you start by choosing a key? So for say if you choose the key C do you just play chords in that key and somehow come up with a song structure? also how do you write the melody etc?
I heard that you should write the music first then the lyrics? Paul McCartney used to write the music first most of the time.

A. Music is like a language, or even a mathematical problem.

Think of it like this, choose a key, lets go to C major. Next, using SS(simple structure) we can write a song fairly easy using simple chord progression. Try using a piano, even if you translate the song into guitar at another time piano is a lot easier to write with.

A song using SS will always have a verse, and a chorus, and will usually have a bridge. To get you started, start with this structure.

Verse - chorus - verse- chorus - bridge - chorus with 1 extra harmonic instrument, or melody with the instrument *adds a climactic feel*

Lets start with a cleft, to explain cleft it's the basic bass, or rhythm we'll use with almost the whole song. Basically, using SS, you'll maintain this 3, or 4 cleft chord progression in the entire song, aside from the bridge. With C major, lets go with a 4 chord progression and say, Aminor, F major, C major, Gmajor chord progression. I suggest you play octave, or rolling octave with this chord progression, keep the chords broken rather than playing them totally not to overdue the cleft harmony.

Next, we need some type of treble melody to play, you can have this melody follow the vocal notes if you like. This is up to you, it's easier if it follows the vocal notes, but more impactful to a song in my opinion if it doesn't. A melody can be very simple and still be great, you can simply play single notes the whole time if you like. Even something like say.... c.g..c.b......c.g..b.c (number of . indicate pause time#

If you play that simple single note melody in cunjuction with that 4 chord #again I recommend broken chords or octive/rolling octive) it will sound good.

Note, this is just an example, pick any key you like, use any chords progression you like. This is all just to give you an idea on how to write a song.

I hope this helps, good luck!