\documentclass[a4paper,11pt]{article}
\usepackage[polish]{babel}
\usepackage[utf8]{inputenc} 
\usepackage[T1]{fontenc}
\usepackage{fourier}
\usepackage{makecell}
\usepackage{rotating}


\renewcommand\theadfont{\bfseries\small}
\renewcommand\theadalign{cc}

\begin{document}

\begin{tabular}{|>{\bfseries\small}l|*{3}{c|}}
\hline
\diaghead(2,-1){xxxxxxxxxxx}{Miesiąc}{Rok}
 & \thead{2022} & \thead{2023} & \thead{2024} \\\hline
Styczeń  & 31203 & 28754 & 30562 \\
Luty     & 28564 & 25675 & 27896 \\
Marzec   & 34612 & 31222 & 32454 \\
Kwiecień & 30896 & 29564 & 31112 \\\hline
\end{tabular}

\end{document}