I need to know how to format a cell so that if for example:

If
A1 greater than or equal to 1
Then
A3 = A2*# from A1

Basically I have:
A1 Qty
A2 Cost
A3 Total

If A1 (qty) is greater or less than 1
Then A3 (total) needs to = A1 (qty) x A2 (cost)

Any help would be great. I know basics but conditional formatting is a little beyond me.