You can get a complete LaTeX document to be in landscape format by adding the ‘landscape’ parameter to the document class:

\documentclass[landscape]{scrartcl}

But sometimes you want to have only one single page in landscape format while the rest of the document should be your normal portrait format. This is very simple to achieve with the package ‘pdflscape’. Include the package in the preamble:

\usepackage{pdflscape}

Then when you are at the point where you would like to insert your landscape page put all the content inside a ‘landscape’ environment.

\begin{landscape}
Hello world!
\end{landscape}

A new page will be started at that point, so it might cause half-empty pages, so think about where to insert your landscape page. But anyway, you should only do such things for big tables or figures preferredly in the appendix.