You get selected to go to Ranger School. Or at least, you need approval from your chain of command. It's a leadership school for, I believe, Sgts and above. First and foremost, getting selected is a challenge in itself, especially in the NCO dominated US Army. Secondly, there's very little you can do to prepare other than what you should already be doing as a leading soldier. The main things tested are coolness under adversity, resilience to sleep deprivation and team work. The physical aspect is only one part of it, and unless you want to try not sleeping for 3 days then seeing how many sets of pull ups and press ups you can manage until you pass out, then I wouldn't worry too much about it yet.