Most songs that have ever written can be broadly grouped into one of several categories; songs written around a chord progression, songs written around a melody, or songs written around a riff:
If you choose to develop the song in a minor key this will give the whole piece a sad or moody feel. While writing in a major key will give you an upbeat happy tune.
Many songwriters will use both major and minor keys within their songs, perhaps choosing a minor key for the verse, and a major key for the chorus, or vice versa. This has a nice effect as is good for the listener as it helps break up the monotony that sometimes results when a song lingers in one key.
Of all the aspects involved with creating new songs, writing a strong melody is undoubtedly the most commonly overlooked and possibly the hardest component of music, particularly with modern pop/rock music. This wasn't always the case because the pop songwriters of the 1930's and 1940's focused only on writing melodies and in many cases it was the basis for a song, with lyrics and chords only added in later to give the song more depth.
Generally, the process of writing a song is much different nowadays because usually songs will be created from a guitar, bass or keyboard riff, or a groove. This is built upon, along with the writing of a chorus is written and the adding of bass lines so that the entire instrumental part of the song has been assembled even before the melody has been taken into consideration.
Most people will focus most of their effort of writing lyrics. While lyrical content is obviously of great importance, a strong melody is absolutely necessary, for without this component; the vast majority of people won't listen to the song long enough to even bother listening to the lyrics.
The vocal melody of the song is what is remembered most people; and in many cases is what makes them like or dislike a song. If melodies are well-written and catchy, people will remember and enjoy the music but if the melodies are carelessly or poorly written and bland, people won't