Writing LaTeX
There are many excellent introductions to writing mathematics with LaTeX, for example Introduction to LaTeX. Below is a quick and dirty example to start with.
LaTeX is the language in which most mathematics and physics documents
are written. Mathematical symbols and equations are written in a
special markup. For example an integral sign is inserted with
\int. The best way to learn LaTeX is by example. Here
is a basic LaTeX file:
\documentclass[a4paper,10pt]{article}
\usepackage{amsfonts,amsthm,amsmath,amssymb}
\title{Simple example}
\begin{document}
\maketitle
\section{Quadratic equation}
We want to solve $ax^2+bx+c$. The solution is
\begin{equation}
x= \frac{-b\pm \sqrt{b^2 -4ac}}{2a}
\end{equation}
\end{document}
LaTeX commands start with a back slash \. Sometimes they
have an argument in curly brackets, e.g \sqrt{b^2-4ac}. Most of the commands in the document above are self-explanatory.
To convert this into a readable document first save it into a file
with a .tex extension, e.g. sample.tex.
Then you need to process it. If you are using GNU/Linux, see
Processing LaTeX with GNU/Linux systems.