How to make \section items numbers show as Roman Numerals like I II III IV..?

Change the representation of the corresponding counter (section, in this case) by adding to the preamble:

\renewcommand\thesection{\Roman{section}}

EDIT: this redefinition will probably require increasing the space allowed for typesetting section numbers in the ToC; this can be done, for example, with the help of the tocloft package by adding the following lines to the preamble:

\usepackage{tocloft} 
\setlength{\cftsecnumwidth}{1.5cm}