Use footmisc package
\usepackage[hang,flushmargin]{footmisc}
The ‘hang’ option flushes the footnote marker to the left margin of
the page, while the ‘flushmargin’ option flushes the text as well. The
documentation displays some other options that you could fiddle with
to obtain the results you require.
from LatexUsersGroup written by Werner Grundlingh