Greenhouses, Sheds and Sun Rooms - Building a Greenhouse

I want to convert our back patio that has a concrete slab and a plastic sheet lined roof into a greenhouse that will be warm enough to grow herbs in the winter. I am thinking about a knee-height surrounding and probably enclose the whole greenhouse with plexiglas.

Is brick good enough for insulation? Should I use other material to make the lower surrounding?
Is Plexiglas a good material for making a greenhouse?
How thick of a Plexiglas do I need to keep temperature constant?
How do I connect the glass walls together and to the roof?
What kind of roofing material do you suggest?
Do you know any place where I can find step-by-step instructions on building a greenhouse?

Thanks!

You might have better luck posting this in the Gardening forum.