The following is a basic format you could use to write any song. Use as much of this as you need.
Decide on a scale. Yes just one. I sing best in D if this is going to be my song I'd write it with D scale notes for the melody and chords from the key of D will give me the harmony. If you do not have any vocalist in mind C is easy - no sharps or flats. OK I want to write a Pop, Rock or Country song so Major scale and major chords will be a good starting point.
• Decide on a chord progression. Yes one of the cookie cutter progressions will be fine to get started. You can flesh it out later. Since this is my song I'd use a I IV V7 I or D, G, A7, D progression.
• Now the rest is chicken or egg. I chose lyrics, chords then melody. You may want to go melody then chords and leave lyrics for last. It's your song do it the way you want. The order you take does not matter all that much. However, you should touch on everything that follows. I'll give the lyrics first method.
• Get the story into verse format. The music I play is based around someone singing the song (vocals) so if I do not have a story to tell there is no reason for the song. OK that out of the way. Four line verse is a good format. You will need three verses and a chorus. Chorus is the hook, what you want them singing tomorrrow. Rhyme or not up to you. If you are writing an instrumental piece you of course will not have lyrics to anchor your verses, etc. The repeating head (tune) is your anchor now.
• You've got the story into verse format, three verses and a chorus. Each verses brings up a thought, discusses it and brings that thought to a close; so the next verse can bring up another thought. Place your cookie cutter progression over the lyric words. This is my first draft approach. Start the verse with the I chord - you are at rest to start so the I tonic chord makes since. To get some interest into the chord progression we need to get some tension into the progression so move to the IV chord near the ending of the first line. Continue with the IV into the second line and near the end of the second line bring in the V7 chord. This increases the tension and acts as a climax. Since we have reached climax quickly end the 2nd line with the I chord. You moved the first two lines from I (rest) to IV (tension) to V7 (climax) and then resolved back to the I chord and rest. Repeat this for the 3rd and 4th line. I like to get two V-I cadences into my four line verse. Might as well use that same format for the other verses and what the heck use it for the chorus - remember you are doing a first draft. To illustrate I've listed the first words of the Gettysburg Address below and placed chords like I mentioned:
D................................G
Four score and seven years ago,
G...................A7............D
our fathers brought forth on this continent
D...........................G
a new nation: conceived in liberty, and dedicated
G ......................A7..................D
to the proposition that all men are created equal.
Recap of Verse format -- one way could be -- first two lines bring up a thought then the 3rd and 4th line of the verse react to what was said in the first two lines and then bring that thought to a close so verse number two can bring up another thought. The chorus contains the overall feeling of the song and is normally repeated at least once.
• Play that progression and recite the lyrics. Move the chords around to where they match with the lyric words. Move them a little one way or the other - your ear will tell you.
• Now it's melody time. I go to the keyboard for this - at any rate - one melody note per lyric word syllable. In the above the word "sev-en will take two melody notes, as will "fath-ers. One note per lyric word syllable is a great help when writing melody.
• Which notes. The chord's pentatonic will give you three chord tones,
for harmonization, and two safe passing notes - more than enough to build a melody that will harmonize with the chords you are using.
Yes your melody notes and your chord notes should share like notes - when they do you harmonize both the melody and the chord line. I find knowing the progression first then
finding melody notes from within the chords lets me keep the chord progression's journey from rest, tension, climax, resolution and return to rest the verse should travel intact. Now I only have to find harmonizing notes for my melody from the active chord or it's pentatonic. Here is what I do. Recite a lyric phrase and see what chord tone notes sounds best over that phrase. Here is Mary Had A Little Lamb in C; notice it's one melody note per lyric word syllable:
C.....................................Dm.......... .....C
Ma-ry had a lit-tle lamb.... Lit-tle lamb... lit-tle lamb.
E...D...C...D.E..E...E..........D...D..D........E. .G...G
• That will get you a lead sheet, treble clef, chords and lyrics. A bass clef would be nice or just leave it as a lead sheet and let the bassist compose the bass line - how the chord tones are played - as he/she feels best.
Sit back open a bottle of your favorite beverage and start on fleshing out your first draft.
That is just about all that is necessary to write a simple song. This may be a little more than you first thought, Take it one step at a time and it'll all fall together. The chords used do two things; 1) move the verse along in the story and 2) if the chords and the melody (treble clef and bass clef) share like notes harmonization happens. Music is made of melody, harmony and rhythm. Rhythm is another story and needs a string all it's own...
Good luck.