You have to use a shader that is transparent and also reflective (the latter is optional, but you'll probably want it).

In the case of a simple smooth glass you can use Flat_Refl,xshad.
If you want a glass with a bumpy surface use Norm_Relf.xshad.

Firstly you have to set a transparency. Set the mode to Normal.

Then adust Opacity to the degree you find appropriate,

To have a reflection you have to choose an suitable environment map.
You can find a bunch of them in [Common]:Textures\Envmap, Choose one that you feel suits your sorrounding room.

The amount (and color) of the reflection can be adjusted by the Reflection Color propery:

Lower the Fresnel Min Value if you want a Fresnel effect (meaning the intensity of the reflection depends on the viewing angle)

Light sources can be also reflected on the surface.  For that adjust the Specular Color and Specular Power (the spreadiness of the reflection) properties.

Use the Normal Map property if you want a bumpy surface.