You have a object selected and you’d like to snap Ruler Guides to both the left and top sides of its Bounding Box. You could drag your Guides one at a time from the Rulers to the side and the top, but there is a faster way. Select the object with your Selection tool and press your Command key (Control key on PC) and drag both the vertical and horizontal Guides at once from the square in the upper left hand corner of your document window (where the Rulers meet). Release your mouse button when the Guides snap to the upper left hand Bounding Box Point.
Take care not to start clicking and dragging on your Guides before pressing your Command (Control) key or you will just reposition the origin point of your Rulers. Double-click on the square (where the Rulers meet) to reset to its original position.