Here you can find few very useful hotkeys which you should familiarize with, if you want to use photoshop efectively

CTRL + N creating new image

CTRL + SHIFT + N creating new layer

CTRL + A select entire layer as selection

CTRL + SHIFT + I Invert selection

CTRL + D deselect current selection

X flips foreground/background color

D default foreground/background color

ALT/CTRL + BACKSPACE fill current selection or entire layer with fg/bg color

CTRL + T turns on Free Transform

CTRL + R shows rulers

CTRL + Z undo