Step 1: learn C++, there is a variety of beginner lectures on the www
Step 2: Have a first look at ROOT, follow the tutorials and examples on the ROOT homepage
Step 3: get some D3PDs, these are files containing preselected physical processes, e.g. ttbar, SUSY, QCD... (ask one of the 'gurus')
Step 4: create some basic histograms/plots with ROOT (maybe some distributions showing your signal and bakground)
(If you are a bachelor student or a diploma student: ask one of the Lehrstuhl experts to do Step 5 and 6 for you!)
Step 5: use Athena to produce your own D3PDs from AODs - the (working) Athena setup might take several weeks!
Step 6: use Athena on the Grid (using Ganga and/or Ganga Tasks) - the Athena+Ganga setup might take several weeks!