Open a file with "w" as a second argument to write to it. Write mode creates the file if it doesn't exist, and empties it if it does, so be careful. f.write() writes a string exactly as given: it does not add a new line, so you add \n yourself:

with open("greeting.txt", "w") as f:
    f.write("Hello\n")
    f.write("World\n")

with open("greeting.txt") as f:
    print(f.read())

print can write to a file too, with file=f. It adds the new line for you and handles numbers without str():

with open("squares.txt", "w") as f:
    for n in range(1, 4):
        print(n, n * n, file=f)

with open("squares.txt") as f:
    print(f.read())