A selection is active when you can see the 'marching ants' around the part of the image that has been selected. This can be saved or applied to a Layer Mask if the mask is created while the selection is active.
Hard edges such as clothing or bodies are much easier to select than soft edges such as hair, and usually need different techniques.
Ctrl + Click on a layer turns it into a selection.
CMD + SHIFT + I to invert a selection.
CMD + D to deselect a selection.
CMD + H to hide a selection, though it is still active. (Though this hides Photoshop for me).
ALT + BACKSPACE fills a selection with the foreground colour.
Tools for Making Selections
The Marquee Tools - Rectangular, Elliptical, Single Row, Single Column.