How do I write the most precise and simple as possible?

The most important things to concentrate on in essay writing are meeting the requirements of the assignment and communicating these ideas you have to your reader in a way that your reader can process them.

The best place to begin, since you already know what you wish to write about, is with a thesis statement, also known as a claim. In one or two sentences, write down the main idea that you want to communicate to your reader. For example, if you want to write about legalizing gay marriage, you might begin with a statement like "Gay marriage and the structuring of a family unit will increase the emotional and economic well being of local communities." In this way, you're showing the reader what you want to argue or prove in your essay.  

Then create an outline. With your thesis in mind outline the first point (also called a topic sentence) you want to discuss. Make sure that this point helps your reader to understand your thesis better. For example, if you've mentioned gay marriage in the context of community stability, then all your points must substantiate this claim. So you could write a paragraph about the economic benefits of gay marriage (increased access to insurance for partners would mean less strain on the medical industry). Just remember that your topic sentences need to directly relate to your thesis.

Once you have a topic sentence (that expresses the main idea of that paragraph), fill in supporting details as evidence. If you start listing anything that doesn't fit with your topic sentence, delete it.

Repeat this until you have an outline that looks like this:

I. Thesis statement

II. Body paragraph #1-Topic sentence

           a. supporting evidence

III. Body paragraph #2-Topic sentence

           a. supporting evidence

Repeat as necessary. By creating on outline, you can make sure that your evidence listed all relates to your topic sentences and that your topic sentences all relate to and further illustrate your thesis statement. Then when you go to write out the essay, you have a roadmap to keep you clearly focused. Good luck!