The easiest way would be to use a polarizer filter, which allows light coming from a single direction but blocks most of the light coming from the other directions.
Depending on your camera model that could be harder to do. Usually bigger cameras have adapters for filters built in the lenses, but some smaller cameras also offer this option, just not all of them.
So you could use to it to capture the light coming from the direction of the scene itself, but block most of the light coming from the others sources behind the glass (the reflections you are seeing).
As Matt Grum noted, if you can remove or block the light from inside you will have a lot less reflections. But even if you don't have lights turned on behind the window, the light that comes from outside may reflect on you and your background and appear on the window (therefore on your shot too).
Again, even if you had no lights behind the glass, the polarizer would allow you to capture mostly only the light coming directly from outside.