=IF(B4>=40.01,B4-$B$1,"0")
so, if the hours worked (cells B4 - B whatever) are greater than 40.01, then you take the hours worked (cells B4 - B whatever) - what you consider full time (40 hours in cell B1).
By putting the $ dollar signs, it'll lock that cell in place when you click and drag the forumla down to other cells.
if hours worked is not > 40.01, then print a zero