Syntax for this function:
LINEST(known_y's,known_x's)
Although there are two extra arguments, in your case you can disregard them. Has to be entered as an array formula. Because the array formula will return two results (slope and intercept), select a range of two cells, type the array formula and press CTRL+SHIFT+ENTER.
Ex. LINEST({1,3,4,4,7},{2,3,4,5,7}) will return m=1.09 and b=-0.79, hence the best fit line:
y = 1.09 x - 0.79
If you have ten points, the y's and x's arrays will contain ten elements each.