\begin{tikzpicture}[scale=1.5]
    \draw [triangle 45-triangle 45] (0,3) node (yaxis) [above] {$y$}|- (3,0) node (xaxis) [right] {$x$};

    \draw [color=red](0,0) -- +(2.5,2.5);
     \draw plot[only marks, mark=x, mark options={blue}] coordinates{(2.5,2.5)}
        node[above, color=blue] {Punkt $(a_1,a_2)$};

    \foreach \y in {0,2,4}
        \draw (1pt,\y*0.5 cm) -- (-1pt,\y*0.5 cm) node[anchor=east] {$\y$};

    \foreach \x in {0,2,4}
        \draw (\x*0.5 cm,1pt) -- (\x*0.5 cm,-1pt) node[anchor=north] {$\x$};

    \draw [color=green, dashed] (-0.1,2.5) -- (2.5,2.5);
    \draw [color=green, dashed] (2.5,-0.1) -- (2.5,2.5);
        \node[green] at (-0.25,2.5) {$a_2$};
        \node[green] at (2.5,-0.25) {$a_1$};
        \node[right] at (0.05,2.2) {\tiny{2. Koordinate}};
        \node[right] at (1,0.2) {\tiny{1. Koordinate}};
\end{tikzpicture}