Elementary OS comes without Minimize button, in order to minimize the windows you can switch windows with Alt+Tab keyboard shortcut, or with Ctrl+?+Up/Down to Maximize/Unmaximize window.
There is a way to add minimize button, to add it we will have to install dconf-editor.
1. To install dconf-editor, open Terminal with keyboard shortcut Ctrl+Alt+t and type following command:
sudo apt-get install dconf-tools
2. Open the dconf-editor from Application Launcher or from terminal by writing: dconf-editor
3. Open in the menu: org > pantheon > desktop > gala > appearance just like in following screenshot:
4. Change the line: close:maximize to close,minimize:maximize just like in following screenshot:
Now minimize button should allready appear in the windows, close dconf-editor and enjoy!