Co-writing songs can be a lot of fun. Two or more people can sure come up with some good material. Even if you are not in the same part of the country (or world), there is always Google chat or Zoom which can be used to bring everyone together. WikiRhymer.com and Thesaurus.com can come in really handy when you get stuck.
Topic
Coming up with a topic of what the story of a song should be about.
Sometimes it is best to just tell the story in your own words.
Hook or Chorus
In your story, do you have a 'funny line' or catchy way of saying something?
This could be your 'hook' or 'chorus'.
Structure
Putting things together and making rhymes out of your story is the fun part.
At this point you can see your song start to come together.
Music and a Beat
Sometimes you have a 'riff' or catchy part you like to play on guitar or piano.
This can develop into the introduction or melody of the song.