The typical is
$\displaystyle{\min_{x\in S} f(x)}$
but I want to view the word "Minimize" in place of "mín"
how can I obtain this?
Using amsmath you can set this up as a starred version of a math operator so that the limits get correctly placed.

\documentclass{article}
\usepackage{amsmath}
\DeclareMathOperator*{\minimize}{minimize}
\begin{document}
$\displaystyle{\minimize_{x\in S} f(x)}$
\end{document}
This solution requires no preamble definitions:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
$\underset{x \in S}{\text{minimize}} f(x)$
\end{document}