First, determine the height then divide it equally so that all rises will be the same. For instance if the height for your stairs is 84", 12 divides equally into this 7 times, you would have 11 steps and 12 risers 7" high. The tread size is something you decide but be sure to calculate head space on the way down.
It was always the norm to make sure all were equal, this has now changed and you will see an odd height on the bottom stair (either higher or lower) in many showhomes.
Once you have the numbers for the rise and treads, mark it out on a piece of cardboard, cut this out to give you a sample stringer and try it out for size where you are putting the stairs.