\usepackage{amssymb, amsmath}    % what is this for

\newtheorem{theorem}{Theroem}    % defines the theorem environment,

%which you can use in the body of your article.

%LaTeX will visually format the theorems.

\newtheorem{definition}{Definition}

\newtheorem{notation}{Notation}

\begin{document}

% Type article here

% Use % to comment

\end{document}