Second off, look to some of your favorite songs to get an idea of how they are structured, i guarantee they will follow the simple formul below almost perfectly. Look at almost any Metallica, Megadeth, Black Sabbath Song, any other metal song, or any pop music song ever concieved.
Anyhow enough about that, this is fairly easy to grasp.
A simple way to structure a song:
Intro > 1x
Verse > 4x
Chorus > 4x
Verse > 4x
Chorus > 4x
Breakdown/Solo/Lead/Horrible Vocal Part > 4x
Another Verse or Solo or Horrible Vocal Part > 4x
Chorus > 4x
Outro...which is usually a repetition of the Intro with some additions and variations.
Now, repetition is the key to each segment of the song.
Depending on the lyrical content it can vary. For a basic example however the repetition will be simple at best...
In anycase the formula works in multiples of 4, and relies on decent riffs with good transitions from one to the next.
Each element is described below very very simply.
INTRO:
The intro has to set the mood of the song so it is essentially a seperate part that is played once.
Though the intro can be massively different from the verse and chorus musically and in it's tempo as long as it fits the mood of the song and there is adiquate transition from it to the first verse of the song.
VERSE:
Every verse should repeat the riff being used at least 4 times, this is assuming that you are in 4/4 time and your verse Riff is at least 4 meaures long...
If you have a 2 measure long verse riff it might be a good idea to repeat it 8 times...
Whatever you do make sure the last time this riff is repeated there is some sort of variation on the ending to it so that it transitions to the chorus.
CHORUS:
Repeat the chorus line at least 4 times...
But on the 4th repeat change the ending to transition back to the verse...
The chorus should be repeated no more than the verse unless completely necessary. 1:1 ratio
In most cases the chorus can be repeated half as much as the verse...so if you repeat your verse riff 8 times then repeat the chorus 4 times. 2:1 ratio
BREAKDOWN:
This is the part where the song starts progressing in a different but logical direction from the Veres and teh Chorus...
Thorw in something either faster or slower to fill time
Repeat that Riff 4 times at least and 16 times at most. This part will get very repetative once past 4 repeats and that's when the idea of having it combined with some sort of lead or solo comes in.
Throw in some sort of appropriate lead or solo, based on the songs feel and mood.
ANOTHER Solo/Riff/Vocal Part/Whatever:
This part is completely optional.
This should be either another type of verse which means the Riff used for it gets repeated 4 times.
While a solo or more vocals can be thrown in.
OUTRO:
This can be abrupt or prolonged.
IF you choose the prolonged route it will probably mean repeating the intro and adding in a melodic solo.
IF you take the abrupt route it will probably mean just hitting a chord and then a sharp cutting off of all noise.
Either way you combine the repetition of each segment of a song, it all comes down to first making sure your riffs go together well, second is that each part transitions to the next farily easily, third that they are repeated in a logical and simple fashion that does not get too repetitous, and fourth that the intro sets the mood.
Follow these simple ideas and you should have no problems churning out decent tunes.
Happy Song Writing.
