def f(n):
    i = 0
    while i < n:
        j = 0
        while j < n:
            print(str(i) + ", " + str(j))
            j += 1
        i += 1

First, group the lines by frequency that they are run.
Let's say that line 2 takes c_1 time each time the function is executed.  Let's say that lines 3, 4, and 8 take time c_2 each time the outer loop is executed.  Let's say lines 5-7 take c_3 time each time the inner loop is executed.  In summary:

2: c_1
3, 4, 8: c_2
5, 6, 7: c_3

How many times is each group of lines executed, in terms of the size of the input, n?

Line 2:  one time
Lines 2, 4, 8:  n times
Lines 5, 6, 7:  n times per execution of outer loop, which is run n times, so that's n^2

So the total run-time is:

c_1 * 1 + c_2 * n + c_3 * n^2

Choosing the highest-order term and dropping constants, that's Theta(n^2).