Huge database - challenges
Get Visual Studio if you don't have it already.
Get Oracle SQL Developer 4.1.3.
Install the SSDT add-in for visual studio.
Install a local instance of GIT for version control. 
Install the MS GIT Plugin for Visual Studio.
Create a SQL Server Database project in Visual Studio for your database.
Make all database modifications using the SQL DB Project going forward.
Use the Datamodeler tool that comes with Oracle SQL Developer to build a relational model of your MS SQL Database so that you can visualize it - yes, this will work, and it's free. 


Other Notes:


Don't include security roles or users in your SQL Database Project.
Do include Linked Servers, and Schema definitions.
Getting a handle on this is not going to happen over night, so don't beat yourself up.
It would be a lot better to have a remote git repo with a local copy that you replicate - but a local repo is better than nothing, and I suggested it because it's faster to get up and running.