[ON] New business owner with some questions

I have just officially opened a small business in Ontario and im really not sure how to go about charging clients tax on the invoice. I am considered to be a small supplier and i do not have an HST/GST number. So i beleive i don't have to... but I could definetely be wrong. A client is asking about tax on the price of the job and im not sure how to answer their question. If someone has and answer or if someone can direct me to how i can find the answer that would be greatly appreciated.