How to remove indentation for all paragraphs?

If you don't want any indent, anywhere in your document, add setlength{parindent}{0pt} in your document preamble. This quick-fix works, but it is better to include the parskip package. It manages the two lengths parskip and parindent together:

usepackage{parskip}% http://ctan.org/pkg/parskip

From the parskip documentation:

Package to be used with any document class at any size. It produces the following Paragraph Layout:

Zero parindent and non-zero parskip. The stretchable glue in parskip helps LaTeX in finding the best place for page breaks.

In addition, the package adjusts the skips between list items. With package option parfill, the package also adjusts parfillskip to impose a minimum space at the end of the last line of a paragraph.

memoir and KOMA-script classes provide similar functionality.