center of a line?, do you mean mid point?, Either turn on your OSNAP and open settings and select Mid point, or type "mid" when you have started a draw command and it will show you the mid point of a line when you hover the mouse in the approx area of the center point
make sure your osnaps are on...you can push SHIFT + right click > osnap settings...then choose the ones you want....the one you are talking about is midpoint......you can also type MID as said before....