You put it back together in the reverse order you took it apart. There is an adhesive out there however many repair shops keep that secret. googled ipod nano repair shops: iRefresh repair
You have was is referred to as a bleeding screen or crack:) This is fairly simple to replace. buy one online, googled: iRefresh repair Digi There's plenty of sites out there!
More than likely the little black plunger under the button has worn off. You can buy one here or look for another place online for parts and repairs. Google: ipodrefresh DigiX
To keep the volume cable from ripping try holding down the motherboard BEFORE you pry the metal plate off. To keep the speaker from ripping off try heating the plate in that area before prying the plate from speaker. I metal spudger is the best tool for this. If your not comfortable with either save your self the trouble and get a pro to do it for you.
I agree you probably damaged the screen. This would be the best possible scenario since its a part you can replace, again! I would have someone else do it this way it's all on them.
I agree with Luis. However I would not try to replace it unless you know what your doing. You can get a online repair shop to do it for around 70 bucks. Try this site ifixit.com or ipodrefresh.com
