How do you write a song?

Considering the number of people queuing up to tell you how to write a book, "how to write a song" seems like a quieter area. But http://robinfrederick.com/learn-how-to-write-a-song/. In terms of tools, instruments, techniques and resources: I think that these are questions to tackle after the initial one "what sort of song do I want to write?". There appears to be a consensus amongst songwriters that the tools and instruments used influence the song written: something you put together with a ukulele and a notepad while sitting on a beach after a long walk is likely to sound different to whatever you come up with after 10 caffeinated hours playing with Band in a Box.

https://musescore.org/.

With my composition students, we use textbooks for classical composition, but for pop music, there really aren't any good ones because there are two main techniques: 1) listen/copy/emulate; 2) if it sounds good, it is good. So, tangibly, take the first idea into account. Listen, listen, listen. What is similar among a few of your favorite pop/rock songs? How are they structured? (Verse, chorus, verse, chorus, bridge, chorus, chorus?). Are the same four chords used in the verses and the bridges? Also, keep it simple, like, really simple. You mentioned riffs. Well, riffs are just riffs, not songs. Implement your riffs, but those are not your verses. Melodies are simpler than riffs. A lot of songs use the same melody in succession, but with the chord changing under it. Example: one measure melody over a C chord, repeat measure over an A minor chord. I know I am giving ideas more than listing resources, but I teach this stuff as part of my job, but I don't use resources other than experience, for the pop stuff anyway. So, did I mention to listen, listen, listen to your favorite songs? Transcribe it, play it in a different key, use the same chords but different melody notes over it, etc.

https://www.coursera.org/course/songwriting a songwriting course (web video) from the Berklee School of Music. It's very accessible, although a little slow at times.

I have the Rikky Rooksby songwriting book for http://www.amazon.com/exec/obidos/ASIN/0879309423/metafilter-20/ref=nosim/ and I think it's pretty good. He also has one for http://www.amazon.com/exec/obidos/ASIN/0879308621/metafilter-20/ref=nosim/, but I've never used it.

There is a truly wonderful book that helps you through the whole process: http://www.howmusicreallyworks.com As for software, GarageBand on OSX is almost free and will let you record your MIDI with ease. Combine it with the above book and you have a good way of working. Another program that is also worth mentioning is Band In A Box: http://www.pgmusic.com

You're principally interested in writing music for songs, right (as opposed to learning to write music and lyrics)? Are you interested in writing instrumental music or only in writing music to go with lyrics? Sorry if this seems dense but I think there's often a big difference between instrumental and vocal music.

I'm primarily interested in instrumental music, but info on vocal music would be good too.

Resources that will teach me how write a song Songbooks. Memorize songs you like. Learn to play them well enough to busk. Then take them apart, one by one, and find out why you like them. What's your favorite part of your favorite song? Why that part? What's the trick? Where is the magic? (How would you unmagic it, defuse it, just by changing one little thing?) Play your favorite songs backwards and upside down and inside out. Fill your head with interesting chord progressions, interesting tricks, musical tools. And you should find a lyricist. Let the lyricist give you encouragement, inspiration, structure. Instead of aimlessly toodling about, suddenly you have a story in front of you, a mood, a rhythm, a scaffolding, a blueprint, a purpose, a schedule. Maybe you also have a musical partner, if your lyricist can play or sing. And if it fails, just tell your lyricist that it just isn't working for you and you want out. Look for another one. (One trick: find online lyrics to songs you don't know. Write fresh music for those lyrics. Now you have music you can repurpose with new lyrics.)