Open more than one Window?

On what app? On Finder and Safari you can use Command+N and get a new window.

If you want a new window in the same application, press Command N or for example Safari > File > new window. If you're referring to seperate applications, just click on the new app and you can switch between them in mission control or Holding down Command and pressing the tab key.

To see multiple screen at the same time, grab a side/bottom or corner of the window and drag it smaller. Grab the window's title bar to move it around.

Regards,

Captfred