I just started doing things with TikZ today and I don't know how to draw a graph which should look like this:

Can someone please help me?
I just started doing things with TikZ today and I don't know how to draw a graph which should look like this:

Can someone please help me?
As suggested @Alan Munn in his comments, your tree can be simple drawn by use of forest package. For example, the code for the first tree can be:
\documentclass[border=3.141592]{standalone}
\usepackage{forest}
\usetikzlibrary{arrows.meta}
\begin{document}
\begin{forest}
for tree={
edge = {Stealth-},
grow = south
}
[ ^{I}
[2^{I},
[21^{I}]
[22^{I}]
]
[3^{I},
[31^{I}]
[32^{I}]
]
]
\end{forest}
\end{document}
Information about forest you can found in the package documentation.