One way of writing your code would be to use the listings package. Your Gaussian elimination algorithm would then be written:
\documentclass{article}
\usepackage{listings}
\usepackage{xcolor}
\lstset{ %
backgroundcolor = \color{white}, % Background color
keywordstyle = \color{blue}, % Keyword style (just color here)
numbers = left, % Add line number to the left of the code
captionpos = b % Caption at the bottom of the listing
}
\begin{document}
\begin{lstlisting}[language = C, caption={Gaussian elimination algorithm}]
for (int i = 0; i < N-1; i++) {
for (int j = i; j < N; j++) {
double ratio = A[j][i]/A[i][i];
for (int k = i; k < N; k++) {
A[j][k] -= (ratio*A[i][k]);
b[j] -= (ratio*b[i]);
}
}
}
\end{lstlisting}
\end{document}
This code gives:

If you don't want any caption for your code, just remove the caption option at the beginning of the lstlisting environment.
If the language was not C, you can always change the language option to the language you are using.