You do not have to define an environment for that.
If you use a starred \chapter, you need simply two (or three) additional lines. Here you will write:
\chapter*{Uvod}
\phantomsection
\addcontentsline{toc}{chapter}{Uvod}
\markboth{UVOD}{UVOD}
where the \phantomsection is needed only if you load hyperref and want to add a bookmark pointing to the introduction.
If you want to automatize, you can create a dedicated command \starredchapter by using the patching macros of \usepackage{etoolbox}:
\let\starredchapter=\chapter
\makeatletter
\patchcmd{\starredchapter}{\secdef\@chapter}{}{}{}
\apptocmd{\@schapter}{%
\phantomsection%
\markboth{\protect\MakeUppercase{#1}}{protect\MakeUppercase{#1}}%
\addcontentsline{toc}{chapter}{#1}%
}{}{}
\makeatother
EDIT: The code shown above for defining \starredchapter does work, along as it is executed (1) before to load hyperref (2) after or without \tableofcontents (which also relies on \chapter*). Hence it is useless, at least !
I guess that there are people on this forum who know a simple way to fix this.
A workaround is nevertheless possible with slightly more patching:
\makeatletter
\let\schapter=\chapter
\let\starredchapter=\chapter
\let\@schapterori=\@schapter
\patchcmd{\schapter}{\secdef\@chapter\@schapter}{\@schapterori}{}{}
\patchcmd{\starredchapter}{\secdef\@chapter\@schapter}{\@schapter}{}{}
\patchcmd{\tableofcontents}{\chapter*}{\schapter}{}{}
\apptocmd{\@schapter}{\addcontentsline{toc}{chapter}{#1}\@mkboth{\MakeUppercase{#1}}{\MakeUppercase{#1}}}{}{}
\makeatother
This save the normal behavior of both \tableofcontents and thebibliography, thought enabling them to appear in the toc and bookmarks. Finally a MWE demonstrating that everything works (in standard book class):
\documentclass[a4paper,12pt]{book}
\usepackage[utf8]{inputenc}
\usepackage[croatian]{babel}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[margin=25mm]{geometry}
\usepackage{etoolbox}
\usepackage{lipsum}
\usepackage{multido}
\makeatletter
\let\schapter=\chapter
\let\starredchapter=\chapter
\let\@schapterori=\@schapter
\patchcmd{\schapter}{\secdef\@chapter\@schapter}{\@schapterori}{}{}
\patchcmd{\starredchapter}{\secdef\@chapter\@schapter}{\@schapter}{}{}
\patchcmd{\tableofcontents}{\chapter*}{\schapter}{}{}
\apptocmd{\@schapter}{\phantomsection\addcontentsline{toc}{chapter}{#1}\@mkboth{\MakeUppercase{#1}}{\MakeUppercase{#1}}}{}{}
\makeatother
\usepackage[colorlinks,bookmarksnumbered,bookmarksopen]{hyperref}
\begin{document}
\tableofcontents
\starredchapter{Uvod}
\lipsum[1-9]
\chapter{Prvo poglavlje}
\section{Prvi stavak}
\lipsum[10-12]
\section{Drugi stavak}
\lipsum[13]
\chapter{Drugo poglavlje}
\lipsum[14]
\multido{\i=3+1,\ii=5+1}{10}{\chapter{\i-th chapter} \section{novi stavak} \lipsum[\i-\ii]\section{drugog stavka}}
\begin{thebibliography}{1}
\multido{\i=10+1}{27}{
\bibitem{Einstein1905photo\i}
Albert Einstein.
\newblock The photoelectric effect.
\newblock {\em Ann. Phys}, 17(132):4, 19\i.
}
\end{thebibliography}
\end{document}
where the \lipsum and the \multido are only used to pad the document.