I want to save some stuff from my computer on my phone. What folder should I paste this stuff over to, I don't see anything like "Documents" or "General storage".
depends on what "stuff" is... if they are photos i would put them under pictures, music under music, ect... if they are just miscellaneous files, then create a general folder and drag them into that
Put it in Google Drive that is in Goggle's cloud that is free for 15GB and you can access it from your computer or from your phone or tablet as long as you are signed into Google.
Yes, you can add a new folder in the nexus without problems and can have a folder on your pc for backing up your data, ie photos, downloaded files, etc.