If you are looking to work seriously with the tool you have to spend time, not just going through tutorials and more tutorials but really understanding what you are doing... I believe that is a lot of work depending on your background.
Highly desirable to have an electromagnetic background (maxwell equations...).
My recommendation is to start with simple examples to familiarize with the interface and steps involved during the modeling process (just few examples).
I use to recommend also knowing about FEM (finite element method) that is the main solver technique in HFSS. This background is helpful to understand why and how boundary conditions have to be fixed and what are the steps involved on the modeling process.
I highly recommend to spent time to understand really how ports work and how they have to be defined (how impedances are calculated and so on as this is critical for your model as your result will depend on that impedance) different types of ports and so on....
It's also important to know about the different types of solutions (driven modal, driven terminal,...), then you will know what is more appropriate for your modeling project.
Then understand what type of boundary conditions are avalaible and how they work.
I agree with itsjwatson that a way to acquire confidence with the software is just by comparing results of simple projects that you can approximate by other means.
If you are just performing examples step by step you do not really understand what you are doing. Take a look to HFSS help, technical notes on how it works (highly recomended)....
Finally, spent time simulating testing software different options (for instance one excersise could be changing between different boundaries conditions and seing if the results make sense...).
be patient!;-)