The marquee tools let you select rectangles, ellipses, and 1‑pixel rows and columns.
Choose View > Snap, or choose View > Snap To and choose a command from the submenu. The marquee selection can snap to a document boundary or to a variety of Photoshop Extras, controlled in the Snap To submenu.
With the Rectangle Marquee tool or the Elliptical Marquee tool, drag over the area you want to select.
Hold down Shift as you drag to constrain the marquee to a square or circle (release the mouse button before you release Shift to keep the selection shape constrained).
To drag a marquee from its center, hold down Alt (Windows) or Option (Mac OS) after you begin dragging.
With the Single Row or Single Column Marquee tool, click near the area you want to select, and then drag the marquee to the exact location. If no marquee is visible, increase the magnification of your image view.