Familiarize yourself with the key of E minor. Play through the E minor scale and play through the chords of the E minor scale. It may be helpful to write out the E minor scale and its corresponding chords on a separate piece of staff paper to use as a reference. The E minor scale consists of these notes: E F# G A B C D E. The corresponding triads are: E minor, F# diminished, G major, A minor, B minor, C major, and D major. The corresponding 7th chords are: E minor 7, F# diminished 7, G major 7, A minor 7, B minor 7, C major 7, and D major 7.
Improvise in the key of E minor. Do this until you hear something that sounds significant to you. You can start either with a chord progression, a melodic line, or both. When you hear something that inspires you, stop playing and write it down.
Build your music composition off of this Idea. If you initially wrote down two measures of a melody, play around with what the following two measure will be. Continue working like this until you have an entire eight-bar musical phrase. You may want to decide on the chord progression before you continue, but you don't have to. You can write out the melody of the entire song first, but it helps to have chord progression to work from. This is where you decide what works best for you.
Decide what your song form will be. A song form refers to how may different sections are in a song and how many times they may or may not repeat. Most popular music is written with verses, a chorus and maybe a bridge. Each section should sound significantly different and usually has an even number of measures. This is a useful tool, for it's easier to map out what your chord progression and melody will be if you have an idea of how much musical 'space' you are working with. An example of a classic song form is: AABA. The 'A's refer to the verses, and the 'B' refers to the chorus. If each section has 8 bars of music, this song would be 32 bars in length.
Play through the chord progression while playing the melody. Experiment with what voicings work best. Write them down.
Write out the music for your song. Use double lined staff paper to write out the treble and bass parts. If you choose to write out a lead sheet, use single-lined staff paper and write out the melody and chord progression.