I usually work through all the numbers up to 10, it doesn't take long after they get the idea and you end up with the first ten square numbers.
1, 4, 9, 16, 25, 36, 49, 64, 81 and 100
It is worth remembering these numbers as they will come up again and again.
Another useful idea is to highlight all the square numbers on a multiplication chart.
This Square Numbers worksheet from Mathsblog does just this.
Know and use square numbers (1) - Know and use square numbers (2)