"Who are you users?" would be the first question to ask. That would help on determining what questions to ask next. For example with most SaaS products, Ford's quote "If I had asked people what they wanted, they would have said faster horses" is very applicable, meaning people often a limited by their knowledge and personal experience and can't see solutions beyond those, so if you ask them what they want you won't hear what would make a good product. In other industries it can be pretty straightforward, if you are old enough you would remember questionnaires your parents used to fill in for exchange of free products (I think it was P&G) answering all sort of questions about what products they use in their daily life from soap to dishwashing liquid, what would they want improved, what's their favourite smell and whatever you can think of (that then went to phone and I imagine now online).
So my advice would be, ask who is your user first. Then pick the next question.