What about us guys who just know alot? There are more of us than guys who know everything.
You should have done some reading by now to figure all that stuff out.
Watch this... it's an hour long, teaches you some basic stuff about setting up a grow room.
http://www.youtube.com/results?search_query=i+grow+chronic&search_type=&aq=0&oq=I+grow+
I would suggest going through it like a lecture in that you could take notes of important things and keep a "grow journal"... mark down what the humidity is, etc..
that's for hydro growing.
Jose Cervantes did a series that relates more to dirt growing here's his series
http://www.youtube.com/results?search_query=jorge+cervanes&search_type=&aq=f
That way your questions will be answered in entirety and likely give you answers to ones you didn't think to ask.
That's why you'll get alot of people telling you to go and do some research first before asking those type of general ?'s. For two reasons (at least) that I can think of. #1, it's pretty basic info that's been covered over and over and will be covered over and over again, and really easy for you to find and learn the basics.
2, Once you have the basics and understanding down, you can ask more informed ?'s.. like I have my Ph at 5.9 in a hydro set up .. is that too high?
once you watch these videos.. you come and tell me the answer to that.
Don't concern yourself with lumens and get drawn into that whole thing. Lumens are the color of the light you are using and they are only a reference as to how humans see light, not how intense the light is from the bulb. when you hear people say "6500 lumens", it's a color of light... has nothing to do with plants..
Basically you want one "cool white" and one "warm white" light for veg. Those are within the spectrum range that the plants can use. (6500K and 2700K I believe).
How big should your space be? How big do you want it? how many plants do you want to do? you can grow in an old CPU tower, or designate a room just for growing.. you tell us what you want to accomplish and I'm sure you can get some input. I would say a closet would be sufficient for you if you are growing out a few plants.
You don't "need" something to reflect light, paint the closet with FLAT WHITE paint if you can't or don't want to mess with Mylar (the reflective stuff) for just now.
Plants germinating should be kept warm. I usually keep mine in the cupboard above the fridge or the one above the stove and leave the light on over the stove, makes it nice and warm and dark for them.
(you can have them under lights before they break the surface for the soil so what you are doing is fine for them)
Air circulation is always important. I'll look it up, a friend of mine Grizzly Adams, (if you read this grizz, post the link if I can't find it?) came up with a great idea for dealing with the issue of venting in a closet, I can't remember exactly how he set it up, but it's pretty slick without wrecking your door.
The ideal temps, and humidity, and how much to water, and how to mix your soil (if you plan to go soil), proper PH levels for your water, and soil will be gone over in those vid's I posted links for.
Good luck and I hope that helped at least get you on the right path.