In the interest of helping out and getting a thread started for you, I'll try and point out some ideas that I think are correct, and hope that if they're incorrect, someone will correct me and we can both learn something from it.
First thing I'd do is figure out what your light sources are. Looks like you've got a handful: the window, two desk lamps, and probably an overhead light (otherwise that corner with the door will be pretty dark. I'd start by shading according to where those lights wouldn't hit.
(I didn't realize there was a window until re-reading -- I thought it was a blackboard, so add it to the lightsources I demonstrate below.)
First, figure out where that overhead light would cast shadows. I'm working on my wife's Netbook, and I hate touchpads, so this is really, really, really rough:
I work in Photoshop, and use layers. I've made this one purple so you can see it more easily. I think there's a rule about warm lights casting cool shadows, and vice versa, so you should figure out what kind of overhead light you have, and where it is in the room.
Back off the opacity of that layer, and you've got a nice clump of shadows that start giving the room some depth. You might want to not make all of these the same opacity. I'm pretty sure the rule is something like shadows get shorter and softer the further they are from the light source. I might be wrong on some of that.
Now, figure out the other light sources. I've done a very basic circle + Gaussian blur on these desk lamps. You can do this much better than me, even if by just having a mouse. Can we trade back computers, honey? She says no! Carry on!
Find where shadows would be cast by these light sources. You can see them in purple again!
Back those off in opacity, but not as much as for the previous shadows. These shadows will be shorter, but darker, because they're closer to the light source.
The computer monitor will give off a soft diffuse light, but fairly weak shadows. I didn't even bother to draw them in here.
These are some very, very basic, rough, and in many cases, incorrect shadows. Were I you, I'd look around for some basic tutorials on light and shadow. DeviantArt might be a good place to start. I haven't even gotten into highlights, or textures here. Hopefully someone more helpful than myself will chime in soon!