Chapter number and chapter title in one line

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.

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}