The best way to become a better screenwriter is to read as many scripts as humanly possible. Of course watching films is oh so important, but reading a script really allows you access to the "floor plan."
Pick one or two of your favorite films and find their scripts. Then print them, read them like a dozen times, breaking them down in terms of plot, structure, character development, plot points, goals, obstacles, etc.
Here is a short list of where you can access free online scripts: