Go talk to an Army recruiter. Join the Army. Volunteer to attend the US Army Airborne School. In Airborne School someone will ask you to volunteer to join the 75th Ranger Regiment.
The Army does award some guaranteed Ranger contracts to soldiers upon enlistment. However, these contracts are very competitive. The guaranteed Ranger contract does not mean that you are guaranteed to become a Ranger. It means that you are guaranteed to get the opportunity to attend the Ranger Indoctrination Program (the selection program to try out to become a Ranger). If you complete the course then you will be selected to serve in a Ranger Battalion.
Rangers have opportunities for a wide variety of military occupation specialties. However, certain positions are much more needed then others. Those specialties are usually Infantry, Communications, Medics, and Fire Support. If these specialties don’t appeal to you, ask your army recruiter if the specialty that you are interested in is available in the 75th Ranger Regiment.
If you are already in the Army, talk to your career counselor or retention officer about serving in the 75th Ranger Regiment.