Have new line between paragraphs, no indentation

Just include the following before \begin{document}

\usepackage[parfill]{parskip}

Use the \noindent command.

Example:

This is my paragraph.

\noindent My second paragraph

Check the following out for more detail: LaTeX:Layout guide

Single line

\parskip = \baselineskip

Two lines

\parskip = \baselineskip
\multiply \parskip by 2

or

\parskip = 2\baselineskip % no shrink and no stretch

Fixed vertical space

\parskip = 1cm % Or 1 mm or 1in, for example

Stretchable and shrinkable space

\parskip = 12pt plus 6pt minus 4pt

Zero space

\parskip = 0pt