If there is a jrotc program at your HS then join it if you want to. It's like joining a sports team or club there's no military commitment or anything you just do different things. jrotc's mission is to build better citizens. rotc's mission is to make officers and you can't join rotc until you're in college (although you apply for the scholarship as a senior in HS).
However, if your school does not have a jrotc program then, to my knowledge, your out of luck. My school didn't so I could be in one. That doesn't really mean much though. Being in jrotc doesn't really give you an edge of getting an rotc scholarship or getting into an academy. It's just like another extra curricular. HQ can't weight it more heavily than other sports because most high schools don't offer it.
If you want to become an officer, join rotc in college (Whether it's afrotc for air force, nrotc for marines/navy, or arotc for army) or go a service academy. If that is what you want to do, then I highly suggest you focus on getting as high a gpa as you can, STUDY!!! for either the ACT or SAT (the higher the score you get the better), and do as many extra curricular activities as you can. Focus on sports because they weight sports much more highly than clubs. Also work on leadership (i.e try to become an officer of a club or captain/letterman of a sports team).
If you want to become an officer those things are the best possible ways to give yourself an opportunity to either make it into an academy or receive an rotc scholarship.