Sysadmin

Hi guys, I recently started working at a company with about 50 people that has grown to large for their current IT setup. They have no documentation or any SOPs. Has anyone been in a similar situation and how did you go about creating documentation, especially when you are new and don't fully understand all of the services they have in place?

Thankfully it's mostly a Microsoft shop and pretty low tech but there are dozens of exchange rules in place that no one knows why they exist or what they do, dozens of SharePoint sites with critical information strewn about them and so on. It's hard to think where to even start and decide what the best way to organize this information will be, and keep in a place a system where we will update it regularly. Any advice would be greatly appreciated.