Use the following hotkeys for general navigation in the Level Viewport.

To:	Do this:
Pan	Middle-drag or Alt + middle-drag
Zoom	Scroll wheel or Alt + right-drag
Orbit (tumble)	Alt + left-drag
Frame a selected object	Press F
Frame all objects	Press A
Look around	Right-drag

You can also right-click and hold to enter game navigation mode, then use the following hotkeys to navigate:

To:	Do this:
Enter game mode navigation	Right-click and hold
Move forward	W
Move left	A
Move back	S
Move right	D
Move up	E
Move down	Q

Tip: To adjust viewport navigation speed in game mode, press W to move forward, then use your mouse scroll wheel to adjust speed. Scroll forward to move faster in the viewport, scroll backwards to move slower. You can also press Shift to increase the speed of the camera while navigating in game mode or press Ctrl to decrease the speed.