Define a function simply so

def hello_world():

    print("Hello World!")

You can add parameters

def hello(name):

    print("Hello", name)

You can return values

def add(a, b):

    return a + b

To call the function, simply use its name and add round parenthesis, for example

hello_world()

To define a function, you need to use the def keyword
A function does not need to have a return statement as it returns nothing by default