i don't understand how to use a variable outside of a function?

[buran]

based on your code:

import random

def test(arg):
    if arg > 3:
        a = 1
    else:
        a = 0
    return a

def main():
    arg = random.randrange(0,7)
    a = test(arg) # here you can use any name, not neccessary to be a
    if a == 1:
        print("I don't know what i'm doing")

more pythonic would be

import random
def test(arg):
    return arg >3

def main():
    arg = random.randrange(0,7)
    if test(arg):
        print("I don't know what i'm doing")

also note the double quotes of the string - otherwise you need to escape the single quotes