I am very new at using Blender and have a simple scene set up with a table and plane for the floor. I don't know what I did, but now the table and plane move together in object mode, and when I try to select them individually I can't. This is a problem because I need to assign different materials to each thing. What do I do to fix this?
I guess you clicked join button in tools while table and chair was selected .It happens a lot if you are messing or trying stuffs in blender.To fix your project. select that object which you joined (in your case that chair and table). then press tab TAB this will lead you to edit mode .
press 'A' to select all everything . now press p and click on "by loose parts"
press TAB again to go back to object mode.
this will allow you to move individual object but we are not done here you will notice something strange may happen when you will move some object
to fix this you need to reset its origin geometry(sounds tricky but it is really easy) now select all the object by pressing shift and clicking on object . now press "SHIFT+CTRL+ALT+C" click "origin to geometry" and DONE!!! hope it help. next time if you mess things up do not save or close blender . press "ctrl+Z" to reverse your last action . happy blending ;-)