How to remove the whitespace BEFORE itemize/enumerate?

That space is governed by topsep. Set it to 0pt.
You can customize itemize/enumerate environments very easily using the enumitem package.

\documentclass{article}
\usepackage{enumitem}% http://ctan.org/pkg/enumitem
\begin{document}
\noindent Some text
\begin{itemize}[noitemsep]
  \item Item 1
  \item Item 2
  \item Item 3
\end{itemize}
\bigskip\bigskip
Some text
\begin{itemize}[noitemsep,topsep=0pt]
  \item Item 1
  \item Item 2
  \item Item 3
\end{itemize}
\end{document}

To set this globally, use

\setlist[itemize]{noitemsep, topsep=0pt}

See section 5 Global settings (p 9) of the enumitem documentation for more details on this and perhaps creating your own list environment that has these predefined settings.

The enumitem package now provides [nosep] to suppress the space above and below lists as well as the space between items. However, this option does not suppress the space above and below the list if you are also using the parskip package in your document.

An alternative to this is to just use:

\usepackage{paralist}

And the compactitem environment:

\begin{compactitem} 
    \item This is entry number one, compact 
    \item This is entry number two, compact 
\end{compactitem}

to see more on this, check: Compact lists with paralist.