There are two secrets.Firstly,a good Instructor.If he's relaxed and not nervous about his Students doing stalls(or anything else for that matter)then this will rub off on the Student(pardon the expression)For most Instructors this comes with experience.
Second,remember you're nowhere near the ground.If you do it wrong the first time which many do(and this is normal,you're doing something you've never done before!),your Instructor will take the aeroplane off you in good time and recover it.
Don't worry,trust yourself and trust him.
In 3000hrs of teaching people to fly(and many have many times this)I only had one Student who resolutely refused to stall an aeroplane.Sadly they never achieved a PPL.
Good luck with the Licence.