\documentclass{article}

\begin{document}

\begin{center}
  {\bfseries Hello world!
  
  \textbf{The quick brown fox jumps over the lazy dog.}
  
  \textbf The quick brown fox jumps over the lazy dog.
  
  I love computers and \tex.
  
  \latex a document preparation system.
\end{center}
  
\end{document}
