There are some useful and time saving shortcuts of Photoshop.
Ctrl+N = New Document
Ctrl+J = To make copy of a selected layer OR to apply "layer via copy" on a selection from a layer.
Ctrl+0 = Fit on Screen
Ctrl+Alt+I = Image Size
Ctrl+Alt+C = Canvas Size
Ctrl++ = Zoom in
Ctrl+ - = Zoom Out
Ctrl+Alt+Shift+E = To merge the composite onto a new layer of its own. Make sure you're on the topmost layer for this.
Ctrl+E = Merge individual layer or selected layer (more than one).
Ctrl+M = Curves Adjustment
Ctrl+U = Hue/Saturation Adjustment
Ctrl+L = Levels Adjustment
Ctrl+Shift+U = Desaturate
Ctrl+I = Invert.
Ctrl+Shift+I = Invert Selection
Ctrl+T = Enter Transform Mode or Open Character Properties Box if you're working with Type
Ctrl+Alt+Z = Step Backward
Ctrl+Shift+Z = Step Forward
Ctrl+S = Save Document
Ctrl+Shift+S = Save Document as
Ctrl+R = Show/Hide Rulers
Alt+Backspace = Fills with selected Foreground Color
Ctrl+Backspace = Fills with selected Background Colour
Ctrl+F = Re-apply the last applied filter
D = Changes the Foreground and background colour to Default color.
X = Changes between background and foreground colour
F7 = Toggles the Layer Palette
F5 = Toggles the Brushes Palette
Ctrl+W = Close Document
Ctrl+Alt+W = Close All