How do you join the US Army Special Forces?

First, you enlist in the Army. You pass Basic Combat Training and Advanced Individual Training (or One Station Unit Training, depending on what occupation you enlist for). Then you have to wait until you make the rank of Specialist (E4). At that point, you can request to go to Special Forces Assessment and Selection. If you complete SFAS, then you'll have the opportunity to advance in the selection process to the Q-Course. Once that's done, you'll be assigned to an SF unit.
First, you enlist in the Army. You pass Basic Combat Training and Advanced Individual Training (or One Station Unit Training, depending on what occupation you enlist for). Then you have to wait until you make the rank of Specialist (E4). At that point, you can request to go to Special Forces Assessment and Selection. If you complete SFAS, then you'll have the opportunity to advance in the selection process to the Q-Course. Once that's done, you'll be assigned to an SF unit.