hudfreegamer: If you plug in an external mouse, it should be fairly easy. Otherwise your left looking around with the keyboard.

Personally, for all FPS games I use

E = Forward
D = Back
S = Strafe Left
F = Strafe Right

Right Mouse Button = Jump
Left Mouse Button = Shoot
Middle Mouse Button = Alt Fire or Melee
Space Bar = Duck
A = Run

Then I fill in any extra functions with surrounding keys (like using C to turn on the flashlight if that option is in a game).

If you don't have an extra mouse, you could probably look around with the keyboard like this:

J = Look Left
L = Look Right
I = Look Up
K = Look Down (or switch I and K if you like the look up/down inverted)

Then use the space bar as fire and assign something else to duck.
I should clarify, I play Duke 3d with the Eduke32 Source Port. It makes it look and control so much better.