You want to iterate through the list, sum all the numbers, and then divide the sum by the number of elements in the list. You can use a for loop to accomplish this.
average = 0
sum = 0
for n in numbers:
sum = sum + n
average = sum / len(numbers)
The for loop looks at each element in the list, and then adds it to the current sum. You then divide by the length of the list (or the number of elements in the list) to find the average.
I would recommend googling a python reference to find out how to use common programming concepts like loops and conditionals so that you feel comfortable when starting out. There are lots of great resources online that you could look up.
Good luck!