Hi. Most of the advice in the comments already posted is sound, but none of them seemed to address HPC. Working on a cluster invites an entirely different bundle of conceptual and practical hurdles (e.g. parallelism, working remotely, industrial-strength shell scripting &c.) Even though I had been programming since I was a kid, I found my crash-course in HPC to be quite challenging; confronted with a new programming model in a new low-level language, it was the first time that I really appreciated what it must be like not to know how to program at all.

So: code as much as possible. As soon as you can possibly stand it, look into MPI4py and start parallelizing your code. Chances are you won't be working with python in HPC contexts, but learning parallel programming at the same time as C or fortran would be needlessly difficult. I would also try to get time on a cluster as soon as you're in a position to use it respectably. Most universities with HPC facilities have an online application for an account, and some sysadmin might take pity on you :) Otherwise, maybe Amazon has some kind of deal?

In the interim, become _very_ comfortable with bash and general command line fu, and a serious text editor. Good luck!