All of the commands are to be executed in Mathematica . Files in Mathematica are called notebooks. The blue brackets to the right of the notebook denote objects called cells.
Cells are the objects in which all commands are executed.
Mathematica has an extensive help system that contains not only the syntax of each command but also examples on how to use it. There is also the Mathematica Book that is included with the help system. Below is the basic syntax for creating a vector field. We first load the package, then we use the PlotVectorField command. Please note the syntax for loading the graphics package, PlotField.