Creation of a Button is simple:
Button sampleButton = new Button();
This will create a new Button without any text or graphic inside.
If you want to create a Button with a text, simply use the constructor that takes a String as parameter (which sets the textProperty of the Button):
Button sampleButton = new Button("Click Me!");
If you want to create a Button with a graphic inside or any other Node, use this constructor:
Button sampleButton = new Button("I have an icon", new ImageView(new Image("icon.png")));