I need help starting my essay in general. I know what I want to write about but I don't know how to start it. It has to be four pages. Only two...

Getting started seems like something that we all struggle with. There are a few approaches you can use to do this, and which you choose really depends upon which is most comfortable. 

While some people need to begin with an introduction, sometimes it is easier to begin in the "middle" of an essay and write a body paragraph or two first.  You say you know what you want to write about already, so you could write a body paragraph or more on your topic.  I see your assignment has something to do with law and politics. So, let's suppose for a moment you are writing a paper on the Democratic candidates for the presidency.  You may not even know what your main idea is about them, but if you write a body paragraph on each, that could be a good way to begin.  As you write, your thoughts on this subject will become clearer to you, and then you will be able to decide what your thesis statement should be. I had a professor who always said "Writing is a means of coming to know."  That has guided me in all of my writing, and it can guide you, too. The idea is that often when we sit down to write, our thoughts and ideas are not clear to us at all, but as we write, they really do fall into place.  Once you have settled on what your main idea is on your subject, you can then go back and write an introduction ending with a thesis statement and a conclusion. I have done this for many papers, and it can work really well. 

Some people really do best beginning with a thesis statement.  A thesis statement states your main idea and the points you will use to support that idea.  So, for example, if you were writing a paper on the decisions of the Supreme Court for the last few years, you may have a thesis statement like this:

The Supreme Court has made a decided tilt to the right in its decisions on campaign contributions, union rights, and eminent domain. 

That thesis statement, which should be the last sentence in your introduction, provides you with an outline to write three body paragraphs, one on campaign contributions, one on union rights, and one on eminent domain. 

How you get going is not important. It is that you do so that matters!  There is no one right way to get an essay started, so see which works best for you. I have used both ways in my own writing, and both work very well.