How do I change the units to meters or create an object that is X mm wide? So I want to make a model for 3D-printing and searched for a guide on how to change the units to metric units first of course, but the guides I found were totally outdated, because there seems to have been massive changes to the UI.
I don't even know where to start in the Manual and got kinda lost in the pages there without finding an answer.

What I want to do:

1) Change the "blender" units to metric units.

2) Display the volume of objects in Blender.

Thank you guys so much in advance for any help!
Just to make it easier for other 2.8x users.

Change your units to metric.
Properties panel > Scene Tab > Units  



To check the volume of objects, enable the 3D-Print Toolbox addon.
Preferences > Addons and click the checkmark.



After it's enabled, you can see the volume here.
Sidebar > 3D Print tab