Not all songs are created equally. The division between songs is based on the type of music you are writing and the genre that you are using. Each of these has specific rhythms, sounds, instrumentation and lyrics that tie everything together to make the best song. Most of these are based on intricate theories that work together to create the best music. Knowing your genre and the theories that are a part of this first ensures that you are able to make the most of the song.
The significance of a song begins with being able to put together the right lyrics, if you are writing in any singer-songwriter genre. It is the lyrics that keeps the song together and acts as the glue. If you are writing lyrics, look at them from different angles. Each angle can be used as you write. One angle to consider is telling a story with the lyrics. A second is considering the emotions or thoughts that are behind the story.
With any angle you start to write your lyrics with, there is one thing to remember: the way that lyrics form the song. Most likely, you will begin with having a verse, then a chorus. The verse will always have new lyrics, while the chorus will repeat. In the chorus, you want to make sure that you have a hook line--the memorable sentence that ties everything together and is often repeated.
Once you have the lyrics in place, you need to arrange the rest of the piece so that it fits together correctly. If you move through other pieces of music, notice that it is not just the lyrics that keep everything moving. There is also an identification of other types of instruments that work to support the song. Most specifically, create an arrangement that includes a low, medium and high set of instruments. These should all support each other and the lyrics. You also want an instrument to keep everything in motion, such as a drum line. This helps to tie the entire song together.
When you have tied together instruments with lyrics, the effect should be a song that becomes effective. This happens through layers of the music that work. Some of the effects will be:
Rhythm: Keeps the song moving forward and gives the general "feel" to the song. Makes the song stable.
Low end: Works as the back bone to the song. Also is a stability factor with the songs.
Middle: This is a guitar, violins, piano and other fillers. These will sometimes keep the rhythm, but can also color the song. This helps to bring out the lyrics and to give the vocalist a break.
High: This is most likely the melody, or the voice. This should drive everything home, and forward, with a main point to the song.
When these are arranged and work together, the song can be completely effective with motion, sound and the main point of the song.
Know that every song is created differently. What really makes a good song is whether it is genuinely coming from you, and what you are trying to get across. If you really want a good song, study your theory and your genre. Pay attention to the instruments that are used and how they are layered. And, with this, look at the ways that the instruments, lyrics and rhythm play off of each other to get the right effect with the music.