I find it easier to write lyrics about something that has happened to me me or about something I feel strongly about.
I had an argument - no - a discussion - with a wonderful singer songwriter called Robin Laing (from Lanarkshire in Scotland.
He was of the opinion that if someone put him in a room with his guitar/pen/paper and told him he wasn't getting out until he had witten a song about a given subject, he would produce the song in a very short period of time indeed.
So he believes that with the correct motivation anyone can write a song about anything.
I know would be locked away for some considerable time. I need the motivation of a strong emotion - It can be something I found hysterically funny through to something I found tragically sad. But if I have experienced it - it makes it so much easier to write the lyrics.
As for the tune - 'borrow' one - its a lot quicker than writing one!
Each year at the Glenfarg Annual Folk Feast in Scotland the have a competition for the most entertaining song on a given theme. It is one of the funniest afternoons of the year and people from all over the place put a huge amount of effort into writing the most wonderfully bizarre and crazy songs. All the door money goes to a nominated charity or good cause.
e.g. The 'Red Red Rose Bowl' was presented to the best song about matters of the heart, and the proceeds went to the British Heart Foundation. We had the 'Scrub A Duck Cup' where the songs had an ornithological theme and the proceeds went to an animal welfare centre for cleaning oiled birds.
Writing songs for a competition of this nature is tough. Everybody complains that they can't thing of anything to write - but surprise, surprise, on the day of the event there are songs by the dozen and all (well most) of them of the highest standard.

So look for the motivation and the songs will come tripping out.