\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
    xmin=0,xmax=3,
    ymin=0,ymax=3,
    axis lines=center,
    axis line style={latex-latex},
    axis equal,
    xlabel={$\operatorname{Re}(z)$},
    ylabel={$\operatorname{Im}(z)$},
    xlabel style={right},
    ylabel style={right},
    ticks=none,
    enlargelimits={abs=0.5},
    clip=false,
]
\addplot[color=black,mark=*] coordinates {(3,2)};
\draw[-,black,thick] (0,0) -- (3,2) node[midway, above left] {$|z|$};
\draw[dashed, black, thick] (3,0) -- (3,2);
\draw[dashed, black, thick] (0,2) -- (3,2);
\draw (2.7,0)--(2.7,0.3)--(3,0.3); % right angle on real axis
\draw (0,1.7)--(0.3,1.7)--(0.3,2); % right angle on imaginary axis
% angle between the real axis and the complex number vector:
% arc takes three arguments, the starting angle, the ending angle, and the radius of the arc
\draw [->, >=stealth, black, thick] (0.5,0) arc (0:33.69:0.5) node [midway, right] {$\theta$};
\draw [->, >=stealth, black, thick] (3.5,2) arc (0:90:0.5) node [midway, above right] {$+$};
\node[above right] at (3,2) {$z$};
\node[below] at (3,0) {$a$};
\node[left] at (0,2) {$b$};
\end{axis}
\end{tikzpicture}
\end{document}