To seek U.S. Army service, you must be a U.S. citizen, U.S. National or a legal resident alien of the United States; if not, you must have an I-551 or G-845 to join the U.S. Army. You may apply for that here: http://www.uscis.gov/portal/site/uscis Army regulations prohibit us from assisting or sponsoring you into the U.S. Once you have an I-551 or a G-845 verifying your approval for permanent residency, you can process for enlistment.

You have to be contracted by junior year. If you can't then, you can always apply to OCS. No, you are not eligible for ROTC scholarships if you are a Green Card holder.

Not if you're looking to commission. The only way that works is if you enlist, then you get the Post 9/11 GI Bill and that. The only ways to get them to pay for it to commission is ROTC scholarship or West Point.