How do I create a function that converts a string to a number(like float). I need to write it by myself

I was given a task of creating a function that can convert an input string to a very accurate number (e10000) to then use in quadratic equation. In short, it is a float function but must be made by myself.

Honestly, I have no idea how to make such a function. I'm only starting and it is my enrolling project for a uni course.

Could use any help, thanks!