You should try posting this to the terrarium forum as well. You will need a small water bottle that you can mist. As for the lid you'll need to use it if you plan to keep ferns and moss to create a humid environment. You'll need to remove the lid at times so it won't get too wet. Using a lid isn't an absolute, but given that it'll be in an office, I imagine the air will be cool and dry.

I don't have a terrarium, but like you I'm planning to start one. I've been looking at moss and am thinking about ordering from Moss Acres. They have a terrarium moss kit.

This is from Moss Acres.

http://www.mossacres.com/terrarium_how_to.asp

Terrarium How To

Terrarium ideas? Think moss.Looking for terrarium ideas? Think moss!

Moss has been around for 350 million years, and thrives in the same humid, CO2-rich micro-environment as that found inside an
enclosed terrarium.

Moss has no true roots, so it
absorbs all of its moisture and nutrients through its simple, yet
elegant, leaf structures. That's
why creating a terrarium with moss is incredibly
easy - simply place the moss inside the terrarium and water.

If kept indoors, terrariums with
moss require more light than you might realize. They definitely need a
fair amount of
relatively strong, natural light, or more regular
artificial light from fluorescent or LED grow lights. Indoors, a large
terrarium featuring moss
should be kept reasonably close to a window.

If kept outdoors, terrariums with moss should be place in a shaded area.

Be sure to use a soil that drains
well at the bottom (add sand and gravel), but also make sure the clumps
of moss are pressed
firmly into wet soil on top to make a good bond -
moss does not do well with air space between it and the soil.