It is normal in primary to be part of a rota for this.
And- it is scary, it was one of the most challenging things when I was a NQT (back in the day) but it has got easier, I quite enjoy it now.
I would see what the others do in CW/assembly if possible- some schools follow a fairly consistent format, e.g. with regards to a song or prayer. Some school also have a scheme for it. Will other teachers/staff be there as well?
If it's a church school then https://www.assemblies.org.uk/ can be useful.
The story/book idea is also useful, and also look at newsround for ideas on current affairs. Children often also like it if you talk about something about something personal, you/your family/your dog etc. I often end up expanding on examples from our scheme with either me or my family, because the children listen and it just seems a little bit more relevant. E.g. I got cross this morning because everything went wrong. We woke up late, the cereal box was empty, Fred couldn't find his shoe, all the traffic lights were red on my way to school. Then I shouted at Mr Smith even though it wasn't his fault. It doesn't have to be exactly true, if you prefer a little more 'space'.
And, if you can manage it/bear it, some form of interaction/acting out helps too, even if it's just to get children up to hold a picture, toy, or a word on a piece of paper.