Primarily, a song consist of a melody (the tune) and accompaniment (chords). This discussion of song writing assumes that you already have some what of a foundation in these applications of music theory. Although this is broken down to simplify the process for you, you may still want to refer back to some basic music theory.
A song is really nothing more than sections of music put together in varying orders. These sections have names (intro, verse, chorus etc ……). The sections are then divided up into measures and the measures are divided up into beats. The beat is the foundational building blocks of a song. Most commonly, measures have four (4) beats in them. A measure can have three (3), two (2) or really any amount of of beats per measure, but probably 95% of the songs we here in music today are in what is called 4/4 time. It is actually so common in music that it is even referred to in music theory as common time and is symbolized by the letter C. Another thing that is a "probably" in music is that most sections of songs are typically even number sets like four (4), eight (8), twelve (12) or sixteen (16) measures. They don't have to be, but they typically are. Below are the beginnings of a chord chart. A chord chart is like a road map of the song. Chord charts are used in studios and practice sessions to map out the song. This is also the beginnings of a songs construction. The purpose for you to see this chord chart is so you can start visualizing what a song "looks" like.
C / 1 2 3 4/1 2 3 4/1 2 3 4/1 2 3 4 /:
The above diagram is a typical 4/4 (common time) 4 bar section. The lines you see are called bar lines. From one bar line to the next bar line creates a measure. Measures are also referred to as bars.Each measure in the diagram contains 4 beats. The two dots at the end of the four bars is called a repeat sign. It means to play that section again.