Ok, here's the deal about becoming a Ranger...
There is Ranger school, which is a leadership school. Then there is the Ranger regiment. They are not mutually exclusive.
Ranger school is open to all Combat Arms Officers and NCO's, and if you go to the Ranger regiment, you will go through the school once you are slotted in a leadership position. Failure to pass Ranger school will mean you must leave the regiment at that point. Ranger school is 8 weeks long broken down into 3 phases. Darby phase, mountain phase, and jungle phase. Don't be fooled by the 8 weeks though, only about 15 to 20 percent of attendees complete Ranger school on their first go. It has a 60 percent failure rate as a whole and the training will usually last 10 to 14 weeks for successful completion. It is a suck fest...you will burn several thousand calories a day every day and only be allowed to take in about 2000 calories a day for sustenance. You will only be allowed 3-4 hours of sleep a day (rarely all at once) and it is not uncommon for attendees to be hallucinating from sleep deprivation and exhaustion by the time they reach the jungle phase. Expect to lose about 20% of your body mass, ruck with 100lbs or more for dozens of miles day in and day out, and expect almost ridiculous uncompromising standards from the RIs. It doesn't matter how good of shape you are in, your body will break down and it will fail...the key is to push through it and force your will to take you to mission accomplishment.
Now, the Ranger regiment has RASP meaning Ranger Assession and Selection Process. It too is 8 weeks long and all members of the Regiment must pass this selection and indoctrination process to become a beret wearing member of the Ranger Bat. It too is a suck fest and tests your will to become a Ranger. The thing about the Ranger regiment is that every time you move up a position within the Regiment, you must go through RASP again, from the Team leader to the Regiment commander. That way, nobody can "hide behind their tab".
I'll mirror what the others have said on here...be prepared to train, train, and train...deploy to the hell holes of the world and have one of the highest optempos in the entire US arsenal. You will be cold, hot, miserable, exhausted, frustrated, more exhaustion, and be held to higher standards than any unit in the US Army...but...you will find that the caliber of men who are in units like this would do it all over and over to be a member of this unit and the professionalism that comes with it.