There are may ways in which a song can be written, this is one of them. Here are some basic steps:
Mostly any song need lyrics. These can be either in a poem form, free form (no rhyme) or other. You can start by simply taking a phrase, a feeling, a situation, a general idea or belief and writing a poem about it. If you do write a poem I suggest keeping the same measure(number of syllables in a verse) troughout the poem. Now if you want a chorus (refrain), choose the most relevant part of the poem and make that your chorus. You can write the chorus separately, or you don't have to have a chorus. But if you do have one, remember that the chorus must reflect the idea behind the song( that and, it has to be catchy).
Tune (or melodic backbone)
This can be done in two ways:
You first have the lyrics; you take a guitar (or a piano) and start playing a tune (a succession of note, arpeggios, or chords), then try to sing the lyrics along with the tune.
You have a good tune in your head; start playing it and then try singing different lyrics that come to mind.
No matter which one you pick, keep in mind that the lyrics and the tune have to represent each other, you must get the same feeling both from the lyrics and the tune. If the tune is soft, choose according lyrics (something regarding love, relaxation, world peace etc.)
Basic song structure
You now have a tune, and some lyrics. You have to decide a basic song structure for your song. This means you have to decide how you're going to split the lyrics, chorus, intro, outro, solo. Here is a good song structure: small guitar intro, first verses, chorus, more verses, chorus, small chorus add-on, guitar solo, chorus, small outro.
The intro and outro can be the same as the basic tune, or just the first part of the tune, but without lyrics. If your playing it on a guitar, it easier to change the chord after each verse.
Adding to the song
If you intend to play the song with a band you have to add to the song. Lets say you made your song tune on a acoustic guitar. You can include the following instrument to the sons:
Bass: the most important thing about the bass is that it keeps a good rhythm and it also completes the frequency spectrum of the song (by the low pitched notes it plays). A good tip for the bass musical line is to mark the crossing between chords(arpeggios) by playing the fundamental note in that chord.
Electric guitar: there are a lot of things that an electric guitar can do in a song. It can help keep the rhythm, it can use special effect to add to the songs feel, it can play a good solo and so on. If you use a acoustic guitar as the base rhythm, don't play the same chords on the electric guitar, it becomes too full. You can use fifth to sustain the tune. You can also play a nice simple tune that goes along with the main tune, but were the electric guitar excels is at solos.
Drums: these are very important to a song, they are the ones who keep the beat of the song. You can have a full drum set, or just a couple of bongos. If your drummer is good, he'll have no problem at getting the beat of the song. You can vary the percussion of a song a lot, so don't hold back.
Piano: this is a very good instrument to base your song on. As a secondary instrument, it can be used for background effects, and certain solo parts. But if you have based your song on an acoustic guitar there is so much a piano can do.
Other instruments: there are many instruments that you can use to complete the sound of your song. Don't hesitate to try new and exotic sounds.
After you tried all the instruments available, and all the variations you wanted, it's time to set the final form of the song. If you're in a band, agree to a common version for the song. Don't forget, you'll have to make compromises some times (that's a part of being in a band). If you want to play it life, rehearse it a few times, just to be sure you all know how the song goes. As a conclusion, you don't have to have a license diploma as a musician to write a song. Just play what you like, be open minded and the songs will come to you. A song shouldn't be written because you have to, a song should be written because you want to.
Here is an example of a song:
I wrote a song on a Sunday morning, (C) (C) (C-G)
I did it till my pen started moaning, (Am) (A) (A-E)
I picked up a guitar and started to play, (F) (F) (F-C)
I wrote my song and I did it my way.(G) (G) (G-D)
Be free, (C) (C) (C-G)
Let yourself be free
Rise to the challenge(G) (G) (G-D)
Of your own destiny.
Chorus add-on: Break the boundaries of society.
I played my song in a thousand places(C)
I brought smiles to a thousand faces(Am)
I faced my fears and I got this far, (F)
I am now a world famous rock star.(G)
The symbols in the brackets are chords (you can choose a chord pattern by using The Circle of Fifths). You change the chord when the verse ends.
a very simple bass line can be done by playing the fundamental of each chord when the verse begins. The bass can peed up the tempo where it is required.
the electric guitar can play fifths ( C-G, A-E, F-C and G-D), can also vary the rhythm, can have a distorts effect on the chorus. It can do a simple solo (because it's the C major scale, you don't have any accidentals(#)) after the chorus add-on (you can use a pattern from Musical Mode Pattern for Picking article).
the drums have a basic 4/4 beat which can be varied easily. Small drum breaks at the end of a verse and the begin of the chorus are a good idea, it tells the people listening that a chorus is coming.
Enjoy. Your guitar community - www.guitaristsunited.com.