The header consists of left, right, and center. So I am not sure this is possible My guess is that you could create a new command like \newcommand{\R}{\section{R}}, but I am not sure that is what you are looking for. I used a \Huge command for the section in the following example.
\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\pagestyle{headings}
\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhf{}
\rhead{\Huge{Section}}
\lhead{Guides and tutorials}
\cfoot{Page \thepage}
\begin{document}
\noindent Hello, here is some text without a meaning. This text should
show what a printed text will look like at this place. If you
read this text, you will get no information. Really? Is there
no information?
\end{document}