Decide whether your song will have an introduction or start with a verse featuring singing. An introduction can be anything from one instrument performing a section of the song solo to all the instruments gradually getting louder before beginning the verse. On the other hand, starting the song with music and singing can make for a striking beginning.
Make the verse the next section and decide whether you want two verses before the chorus or one. Having two verses can give the chorus more impact when it is introduced, but it could make the song overly long. The verse is the narrative element of the song and usually features more lyrics than the chorus for that reason. Also, the music is usually less showy or memorable than that of the chorus and serves to backup the singer's voice.
Decide if you want a pre-chorus in the song. This is a moment where the music intensifies for a bar or two in anticipation of the upcoming chorus.
Introduce a memorable melody in the chorus and lyrics that are easy to remember and easy to repeat. The chorus is the section which can stick in people's minds, so a clever lyrical turn of phrase will also go a long way in ensuring that your song is memorable. The lyrics should also summarize or expand upon the theme of the verse lyrics.
Listen to the song so far and decide if the song needs a bridge to add a bit of variety to the song. Most songs follow a verse-chorus-verse-chorus structure so a bridge will usually fit in well after the second chorus. The music in the bridge should bare some resemblance to the rest of the song, but could change key or could feature different chords. It could also segue into another pre-chorus followed by a final chorus.
Decide how you want the song to end. A fade-out is when the music continues, but the volume is slowly lowered to end the song. Alternatively, you may want a musical "outro" to end the song, much like the intro begins the song. Again, this could be either an instrument performing the melody solo or the whole band gently slowing down and letting the final note ring out.