You find the entry in the body of the table that says 0.975, and read off the correcsponding z value that will give this cumulative probability.
If the value did not occur exactly you would either choose the closest value that does occur or interpolate in the table to get the required point.
2. Using the right hand table
Look up the z value for 0.975 (the values in the body of the table are z values.
We are using 0.975 because the critical value of z cuts off an area of 0.975 to its left, but you want the two sided case with 0.025 above and 0.025 below the cut offs, but these are symmetric.