How to add custom fields to registration page ?

Is there any plugin that will allow me to add custom fields to my registration page ?
Like : age,date of birth, city,sex…. ?

I am looking for plugin because I dont know how to manually add these fields.
I found few tutorials but they dont help at all.
They just say that I should input some code in my functions.php file but they dont say where, at top, bottom or somewhere else.