Inops Answers for Xbox One

Which button fires?

Never used an xbox or anything similar before but now there's one in my living room so I thought I'd try. Chose inops because it is free and pretty. The game tells me how to split/collect and jump, and the joystick on the left steers it. It tells me to hit the ignus when it's eyes are white but I don't know which button to press. I try hitting random ones but have always got killed before anything worked. Google is not being my friend. Please imagine you are explaining this to your granny's granny and you'll be pitched about right.