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.
SHIFT + DRAG when using the Rectangular Marquee Tool keeps the selection square. Similarly when using the Oval Marquee Tool it keeps it circular.
SPACEBAR + DRAG moves a selection around.
SHIFT + Marquee Tool changes the curser to add another selection.
SHIFT + - KEY changes the curser to delete a selection.
Press ALT when using a Lasso Tool changes it to a Polygonal Lasso Tool and vice versa.
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).
OPTION+ BACKSPACE fills a selection with the foreground colour.
BACKSPACE deletes the selected pixels.
Q enters Quickmask Mode to show in red what you have not selected.
Tools for Making Selections
The Marquee Tools - Rectangular, Elliptical, Single Row, Single Column.