Question: Using this preamble:

```
\documentclass[pdftex,11pt,a4paper,openany]{memoir}
\usepackage[brazil]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{indentfirst}
\usepackage{latexsym}
\usepackage{amsmath,amssymb,amsfonts,wasysym}
\usepackage[pdftex]{graphicx}
\usepackage{wrapfig}
\usepackage[left=3cm,right=2cm,top=3cm,bottom=3cm]{geometry}
\usepackage{color}
\usepackage[dvipsnames]{xcolor}
\definecolor{darkgray}{gray}{0.3}
\setcounter{secnumdepth}{3}
\setcounter{tocdepth}{3}
\usepackage[pdftex, colorlinks=true, linkcolor=darkgray, citecolor=blue, urlcolor=blue]{hyperref}

```

TexMaker gives this error message:

```
! LaTeX Error: Command \newfloat already defined.
Or name \end... illegal, see p.192 of the manual.

```

Where is the problem?

Answer: 
You can use the `enumitem` package. If you don't want to use additional packages, you can simply redefine `\labelenumi`:

```
\documentclass{article}
\usepackage{enumitem}

\begin{document}

\begin{enumerate}
    \renewcommand\labelenumi{\bfseries\theenumi}
    \item First item.
    \item Second item.
    \item Third item.
\end{enumerate}

% requires the enumitem package
\begin{enumerate}[label=\bfseries\arabic*]
    \item First item.
    \item Second item.
    \item Third item.
\end{enumerate}

\end{document}

```

EDIT: on a more programmatic level, you could define a new list-like environment that behaves like the standard `enumerate`, but with the desired format for the label. This can be done using something like this

```
\documentclass{article}

\newenvironment{boenumerate}
  {\begin{enumerate}\renewcommand\labelenumi{\textbf\theenumi}}
  {\end{enumerate}}

\begin{document}

\begin{boenumerate}
    \item First item.
    \item Second item.
    \item Third item.
\end{boenumerate}

\end{document}

```

or imitating the definition of the `enumerate` environment as given in `source2e`):

```
\documentclass{article}

\makeatletter
\def\boenumerate{%
\renewcommand\labelenumi{\textbf\theenumi}
  \ifnum \@enumdepth >\thr@@\@toodeep\else
    \advance\@enumdepth\@ne
    \edef\@enumctr{enum\romannumeral\the\@enumdepth}%
      \expandafter
      \list
        \csname label\@enumctr\endcsname
        {\usecounter\@enumctr\def\makelabel##1{\hss\llap{##1}}}%
  \fi}
\let\endboenumerate =\endlist
\makeatother

\begin{document}

\begin{boenumerate}
    \item First item.
    \item Second item.
    \item Third item.
\end{boenumerate}

\end{document}

```

Something similar, but now with the help of the `enumitem` package:

```
\documentclass{article}
\usepackage{enumitem}

\newlist{boenumerate}{enumerate}{4}
\setlist[boenumerate,1]{label=\bfseries\arabic*}

\begin{document}

\begin{boenumerate}
    \item First item.
    \item Second item.
    \item Third item.
\end{boenumerate}

\end{document}

```
You should use a package like `enumitem`. The package allows to modify the `item` by a key-value-syntax.

```
\documentclass{article}
\usepackage{enumitem}
\begin{document}
\begin{enumerate}[label=\textbf{\arabic*},start=0]
   \item First item
   \item Second item
   \item Third item
\end{enumerate}
\end{document}

```

![enter image description here](https://i.stack.imgur.com/2xSBp.png)