\documentclass[]{article}
\usepackage{amsmath}
\usepackage{amssymb}
%opening
\title{}
\author{}

\begin{document}

\maketitle

\begin{abstract}
This is my abstract.
\end{abstract}

\section{Method}

In Section \ref{sec:Introduction} we introduced the problem.

\subsection*{Numerical Method}

\subsection*{Analytical Method}

\section{Introduction}
\label{sec:Introduction}

The typesetter doesn't see    spaces
in between       words.
I start a new sentence on another line,
again the typesetter doesn't notice. If I want a new paragraph, I have to leave a blank line before starting the next sentence.

Now I can start a new paragraph. Using a line break like this \\
IS NOT A new paragraph. You will almost never need to use the two backslashes in normal text.

\section{Maths}

We can write maths in line using a dollar sign $ x = y $. We can write the mathematics in the center of the page like this 
$$
ax + by = 0.
$$
The equation of the line is
$$
y = mx + c
$$
where $m$ and $c$ are constants. If I want the equation to be labelled, I need to put it in the equation environment. We need a begin and end command like this:
\begin{equation}
a + b = 5.
\label{eqn:simple example}
\end{equation}
In (\ref{eqn:simple example}) we give an example of the labelling and referencing of equations.
Some more complex mathematical expressions:
$$
\int_0^{ e^{l^2}} \left( \frac{x+y}{z} \right) dz
\pi\nleftarrow
$$
\end{document}
