I'll answer these in order. If you go to West Point, you won't be in the Army, but if you stay in WP then at the start of your 3rd year you owe the Army time so if you fail out after that, you'll go active as a Specialist and serve some time to pay it back.
If you go to a normal college then you you can come in as a 2LT after ROTC, but wait! Most officers that are making a career out of the Army and want command pick Infantry and Armor branch due to the advancement oppertunities. Especially if you want to go Ranger. Say the Army only decides to commission a certain number of officers this year.. say 10,000 (that's an abstract number, I'm just making an example). Say the graduating class from WP is 4,000 (again, these are fake numbers for example purposes). All 4k of those guys get comissioned automatically. Then say that another 2,000 guys went Green to Gold program or Officer Candidate School from the regular Army.. those guys get in guaranteed as well. That leaves 4,000 spots left. They make those spots up from ROTC guys, but guess what? The top WP, OCS, and G2G guys already got to pick what branches they wanted. Soooo, you may not get picked up at all, or even if you do it might not be for the branch you want. ROTC is not a sure thing, but West Point is, see?
Highschool ROTC could get you PFC rank on the way in, but you are going to wait to at least Corporal or more likely Sergeant before you will command anything. If you come in as an officer, the lowest amount of guys you will command is as a platoon leader (could be anywhere up to 60 guys if you go infantry), but you will have a Sergeant First Class right next to you mentoring you and making sure you don't do anything stupid.
Lastly, Rangers are their own thing. If you come in wanting to go to Rangers, you will do 11 series OSUT training, followed by Airborne (usually) followed by RIP (Ranger Indoctrination Program). IF you pass that, then you will go to a Ranger Batt to "grow up". If they like you and want to keep you in the Rangers, they will send you to Ranger School to get your tab, otherwise they will just have you reasigned to a normal infantry unit. That's if you come in enlisted. Officer spots in the Rangers are very competative, so if you are planning on being an officer and leading Rangers, you better be one tough mofo because there are a lot of guys trying to get a small number of those highly coveted positions, and it will not be easy.