Chapter number and chapter title in one line

If you use a KOMA-class you will not need the package titlesec:

\documentclass[chapterprefix=false]{scrreprt}
\makeatletter
\renewcommand*{\chapterformat}{%
  \mbox{\chapapp~\thechapter\autodot:\enskip}%
}
\makeatother
\begin{document}
\chapter{A chapter}
\section{A section}
\end{document}

You don't say which document class you are using, but for the standard classes, you can adjust these parameters easily with the titlesec package.

\documentclass{book}
\usepackage{titlesec}
\titleformat{\chapter}[hang] {\normalfont\huge\bfseries}{\chaptertitlename\ \thechapter:}{1em}{} 
\begin{document}
\chapter{A chapter}
\section{A section}
\end{document}

You can change the font size as needed, and if you need to change the spacing, there is also a command for that.