How to close apps on Macbook Air

I can't figure out how to close "apps" in my new Macbook Air. The help guide says that you are supposed to click on some menu or something at the top of the screen, but when I had nothing open on my computer, I was unable to eject a flash drive because it said it was still in use by "pages." I am completely confused because I could not find where "pages" was open anywhere. This is my first time using a Mac, so it is a bit different from a PC. Also, my brand new Macbook is also suddenly running slow. In addition, when I go to a website and click on an icon for something, there is no response. It's like my computer is dead or frozen. When I refresh the page or close down the page and login again, it only works some of the time. I am NOT happy with my Macbook Air, and may be returning it since it is hard to figure out.