Yes, it's always difficult to come up with something new - that youtube link to Pachelbel's canon is a prime example. That chord progression is so old, you'd think no-one would use it any more - but it still keeps cropping up!
One song he doesn't mention, that uses the same chords, is Streets Of London - the old Ralph McTell hit.
It's a good idea, when started to write, to stick to subjects you know well - the obvious one is YOU! The way you feel, the way you felt, the way someone made you feel - then expand on that. Why did you feel that way?
I usually start with a phrase that sounds good - might be something I've heard or read. that'll probably be the hook - then I'll write around that. A dictionary's a useful tool, a Thesaurus even better - gives you lists of related words.
But the main thing is, it's got to mean something to you - and therefore your audience. They have to think, "Yes, that rings a bell, that happened to me...." that's where good songs come from. from the heart, from experience, from life.
And songwriting, like playing guitar, is something you can practise. Open a book at any page - find a phrase or a line that appeals to you. Then try and write a song about it. Most of the time, you'll come up empty - but you may come up with a line or lines that you like and might want to keep for a future song. Carry a small notepad and pencil down - write down ideas that you might have. Nothing worse than having a killer line in your head and losing it because you were distracted!
Songwriting's like any other musical discipline - it has to be practised and honed. Work at it, and in time you'll see the benefits!
Vic