How do i be a range candre ?

Im currently serving in Iraq as infantryman. I have 18 months until ETS, these people are telling us that we might get sent to 5th Stryker Combat Brigade if our ETS is later than 6 months after we get back to Fort Lewis. So they are telling us to reenlist now if we want to get out of it, either extend to stay with 3rd brigade or go somewhere else. I think its a load of crap anyway. But im going to get married in 2 weeks when i go on leave and im thinking about staying in the military and go to school at the same time. Can anyone tell me how can i be a range cadre? I heard it was nearly impossible to get in, specially if you are an e4