How do you compose a rock/pop song?

Here are a few websites that might help you out:
http://www.mtrs.co.uk/popsong.pdf
composing pop songs : compose jazz song (http://www.aboutmusictheory.com/composing-pop-songs.html)
Make hip hop music, create your own beats, rap or pop song with Hip-Hop Starz Record Producer and hiphop Jamz (http://www.hiphopstarz.com/)

In addition to the fine answer above, it would help to know how far that you want to go with the composing of the song on your own. If you want to just write lyrics, then you study how to write lyrics so that they fit into a rhythm and a beat. Looking at different types of poetry and practicing writing the lyrics should accomplish that.

If you want to also compose the music, then you are going to need to know a lot about music theory and composition as well as how to compose music for various instruments that might be used as the accompaniment to your song. If you don't want to compose the music, then you could have someone else do it for you.

I am a professional multi-instrumentalist as well as vocalist who also composes and arranges various types of music as part of what I do for a living.