For this article some short ideas how to kickstart a song.

A song contains of a lyric, melody, harmony and rhythm.
It is possible to start a song with each of those elements on their own.

Before we break down each of those four elements let us have a quick look at style to see what kind of impact style has on those four elements: 

Dance music: typically reduces lyric to a very short idea, harmony is usually reduced to a short riff or a few chords. 
Rock and Soul music: Strong on rhythm without compromising lyric, harmony and melody.
Folk music: Think of early Bob Dylan: Harmony is often simplistic, while lyric may be more important, think of the story the song tries to tell.

          Lyric: 

To start a song with a lyric can give you a sense of where you want to go with a song lyrically. You can write down some ideas, or even a short story, or a poem, which can serve as lyric lines for your melody.
Instead of having a clear idea of the words you can also work with “nonsense lyrics”: just use anything which come into your mind to serve you as a melodic idea. Once a song is happening work on your lyric to replace the nonsense words. Sometimes it may be quicker to use nonsense words, it can also give a sense of “non-commitment” Too much commitment at the start of a song may make it harder to get going. The fun element of the nonsense words may help you to open up your creative potential.

         Melody:

Start out with a melody before you even have an idea of harmony. Sing (or play) the melody to yourself. A strong melody can stand on its own. Having a strong melody may improve the nature of your song. Older songs of the past (1930s up to early 1960s) used to be stronger in melody.

        Harmony:

The “guitarist way” of writing songs? Typically mess about with chords, riffs or a handful of notes to see what comes about. It is a very satisfying way to write songs but does have a few downfalls: If you use no other methods you may end up writing the same kind of songs over and over again.  Having chords first may reduce your melody, you may just create melodies to fit your chords instead of the other way around. Ideally you should try to get chords which support your melody and at the same time sound interesting when played on their own without a melody.

         Rhythm:

Work with a bassline and beats first to kick start a song idea. A bassline may open up the harmony, since the bassnotes can be harmonised in different ways. You could start with this method and once you have a groove happening with some chords you could add chords and start thinking about song structure.  The drum/bass method is not just for people who work in the dance field of music, it is just another way to open up your creativity and to get you to think in another way to get a song going.

Try any of the above methods, use them when you get stuck with your own writing, think where you want to go with a song and try various ways of writing.
Songwriting is diverse, and you do not have to plan before hand. Sometimes it helps, the overal process is chaotic, the endresults are neat and, usually, end up with a finished song.

Have fun and hope to catch you soon again,
Eddie