Hope you have insulation and heat in that shed! Working in the cold is never fun. I also hope your drain and water lines are below the frost line... or else you're really looking for trouble.
Somehow, I think turning a garden shed into a darkroom is a bit extreme. But here are some tips anyway.
For ventilation, I always wanted to use a kitchen exhaust hood (the type that goes over the stove) over the trays. It would get the fumes out before they reach your nose. Right now, I have a bathroom vent that fills the need.
For lights, go for incandescent on a string. Fluorescent lights emit a green glow long after they're turned off. You might want to rig the string like in a bus, that way, you can turn on and off the light from anywhere in the room.
Make sure you have at least a 3 foot wide corridor to move around. A good stool to sit on (takes less space than a chair). Make the counter height to
your standards while making sure your enlarger won't hit the ceiling (I'm relatively tall and regular counters are usually a bit low for my taste). Put vinyl floor down, it cleans up easy.
Now, the recommended reading material:
Kodak's Building a home darkroom (publication KW-14 ISBN 0-87985-273-9) a very well made book like all Kodak publications.
The Darkroom Source
http://www.open.org/~hughesa/links/darkroom.php or
http://www.darkroomsource.net/Claudio Bonavolta's photo pages
http://www.bonavolta.chHope these will help transform that small shed into a nicer place.