The standard curve is a plot of absorbance on the y-axis versus the amount of standard protein (in µg) on the x-axis. You can fit this plot by linear regression (if it is linear, as it should be), which will give you the linear equation y=slope*x+intercept.

When you have measured the absorbance of your protein sample, convert the absorbance to an amount of protein using the rearranged equation x = (y - intercept)/slope. Finally, convert µg to mg/ml by dividing by the volume of the sample in µl that was put into the assay.

Thanks!