How can i solve linear system by graphing y = -1

You haven't presented a whole system to solve.  y = -1 is the equation of one graph, and this graph is the horizontal line y = -1.  Graph this line.

Now supposing you have a 2nd linear equation, 3x - y = 5.  Solve this for y:

y = -3x + 5.  Graph this line on the same set of axes as you used for y = -1.
Plot the y-intercept, which is (0,5).  Starting at (0,5), move 1 unit to the right and then 3 units down.  Connect the 2 points with a straight line.  

You must now determine where the graphs of y = -1 and   y = -3x + 5 intersect.  Sometimes this is easy to do visually.

Suppose that you see the 2 graphs intersect.  Determine the x- and y-coordinates of the point of intersection.  THAT is the solution of your system of linear equations.