What do I say about why I left my last job? - what worked for you?

I have an interview next week and I am unsure how to explain why I left my last job as an electrician's helper. This is my first time dealing with any of this. I have been practising in the mirror. Do they ask follow-ups?

Thanks for writing this out. I went through something close to it 3 months ago, and it settled down once I kept a simple log of every call and form.