function help (totaly beginner)

[gruntfutuk]

At least you've made a start and put some structure in place.

def stars(s):
    letters = len(s)
    n = letters-1
    for i in range(letters):

You don't need to check the length of a string in Python for this because you can do a loop using for character in s: and each time the loop goes round, the variable character (in my example) will hold a copy of each successive character in the source string s. (Note that generally we avoid single letter variable names as they are not very informative.)

Before the loop starts, create a variable pointing to an empty string, e.g. starword = "", and then each time through the loop you can add the character and a star. When you have finished, you can return the final new string with the return statement: return starword.

def main():
    word = input("write a word ")
    print(word)

Ok, so this gets some input and prints it out. You need to add a call to your function before the print out to give a new version of word in it with the stars that you want added. You could do word = stars(word).