Latex standard styles, like article or book, do not indent first paragraph, but babel package change this behaviour. For example, it indents all paragraphs when using spanish, even those after a heading.
If you want the first paragraph not being indented, you can comment out these two lines at your language file:
/usr/share/texmf/tex/generic/babel/spanish.ldf ... % \let\@afterindentfalse\@afterindenttrue % \@afterindenttrue
Of course, you can always manually remove indent starting any paragraph with '\noindent', or disable all paragraph indentation with '\parindent=0cm'.