This reminded me of an interesting free reference about assessing carbon footprint:
http://www.bsigroup.com/Standards-a...p-you/Professional-Standards-Service/PAS-2050
Of course you really need to start prior to assessing carbon footprint by evaluating what the intentions are, and what you should mean by "green." If it's nothing but a company PR campaign with no functional requirements or end point that's not such a bad thing; your options are wide open. You might even use good judgment and do appropriate research and eventually take functional, relevant steps that make sense. As already replied if you're out to prove you're green then ISO 14001 is worth considering, or maybe just for using as reference.
I reviewed a book about Green Service Management that went all over the place in terms of what "green" meant, which was fascinating. Not using as much paper is one simple step that's been discussed here, and something simple like changing printer font can reduce toner use.
Of course it also depends on your business, even though that's only a part of where to make a difference. Random Google searching might help, or asking this same question on forums that relate more closely to that topic, or joining LinkedIn groups that relate, etc.