The first thing you need to do is install a TeX distribution. For Windows, look at MiKTeX. For Macs, consider MacTeX. Linux users should install whatever is available through your package manager (it will probably be a variant of TeX Live).
The second thing you need to do is to try a LaTeX tutorial. There are many on the web. There is also a handout used for the Introduction to LaTeX workshops offered at Notre Dame.
If you would like a hands-on introduction, Introduction to LaTeX workshops are offered a few times each year in the Hesburgh Libraries' Center for Digital Scholarship.
The Center for Digital Scholarship offers workshops for those who are completely new to LaTeX. They start at the very beginning and go through adding text, special symbols of LaTeX, and basic formatting. The goal is to learn enough so that participants can find and make sense of tutorials and examples found online.
The workshops are a great way to get started, especially if you prefer to learn by doing.
The handout used in the workshop is also available. Feel free to use it as a reference or to work through it on your own.