Both your questions leave room for imagining what you might really mean, but here's a shot at answering them.
If you have a mask that you want to erase portions of based on an image... assuming the mask is a simple rectangle you drew with the rectangle tool, change the opacity of the rectangle's fill in the color panel to make it see-thru, place the mask over the image, and using the eraser tool, erase the portions of the mask that you do not intend to retain using the image below it as a guide.
As for drawing a circle, hover over the rectangle tool and a menu should appear. Select the oval/ellipse tool. To draw a circle, hold down the shift key and draw just as you did for the rectangle. The shift key forces it to retain equal width and height.
Yes i know what to do that. but what i would like to know if it is possible to erase a portion of the rectangle with the exact shape of the image. not by manually erasing it.