Hi,
take a look at our histogram and TTree tutorials: https://root.cern/doc/master/group__Tutorials.html .
They show how to read data from a ROOT file and plot histograms.
Alternatively RDataFrame offers a higher-level interface to do the same (tutorials here).
For a quick overview of some different ways you can read ROOT files in ROOT, you can check out this previous post, or also search the forum.
Cheers,
Enrico