Learn the basics of music theory (see Resources) as a guideline in your composition. You can write a simple song with just a few key pieces of information. Music is made up of individual notes and chords, and music theory teaches you which ones sound nice together. Firstly, all music has a key, which can simply be described as the dominant note, and if you play other notes in that key, the song will feel like a unit.
Choose a key you wish to compose a song in. This is absolutely down to personal preference, but for a basic starting point, the key of C is a good choice. This makes the C note or chord the "root" or "first." From there, you count sequentially through all the notes in the key of C to identify which notes will go well with it. D would become the second, E the third, and so on in that fashion until you reach the seventh, B. A general rule is that the first (or root), fourth and fifth sound nice together. In the example, this would be C, F and G. A basic run of chords, in the key of C, would be two bars of C, one bar of F and one bar of G before repeating.
Learn about minors and other variations on chords. Each first has a related minor, which is the sixth. In C, it is A minor. If you play a C next to an A minor, the C will sound happy, and the A minor will sound similar, but with a sad tint. Try to tell a story through your music. Perhaps you could start using all of the related minors in the key of C (A minor, D minor and E minor), creating a sad brooding opening, which then builds and switches to the majors.
Start to compose your song. If you are using an online composer, these tend to have easy to use controls. Jam Studio even allows you to choose a key and presents all the related notes to you to choose from. If you are working at home, learn to write musical notation (see Resources), or alternatively, compose directly on your instrument. Experiment with different combinations of notes, and remember to layer multiple parts into your piece. For example, you could have a simple repeating bass-line of E, A, B and E, and then layer a melody over the top in the key of E.