Activity Monitor can be daunting. Try pressing command and tab simultaneously. The icons for any running programs will appear on screen. Continue holding down the command key while pressing tab. This will cycle through icons representing the open programs.
When you reach a program you want to close release the command and tab keys. Pressing Command and Q simultaneously will quit that program. You can repeat this process for any open programs quitting them as needed.
Not visible in doc. Everything seems closed. Saw how to open activity monitor but just saw a bunch of weird text. Running OSX. Isn't that Yosemite?