The geometry package

The geometry package is extremely useful when you want to change the margin sizes (and many other page parameters). It has numerous options—consult its documentation—but we will show a simple example:

\documentclass{article}

\usepackage[showframe,
            paper=letterpaper,
            left=48pt,right=3in,top=4cm,bottom=70mm]{geometry}

\begin{document}
This example document is meant to be printed on US letter paper with
the (atypical) margins shown using a frame.  The header is empty, but
the space for it is shown; the footer contains only the page number
(which is the default).
\end{document}

Notice that too wide text column may make reading more difficult, so think twice before you make smaller margins! Also, when you submit a paper to a journal, don’t bother with margins, fonts etc.—most probably, the journal editors will throw out your settings and change them to the journal style.