If you have never served in any branch of the military, there are two enlistment options. The first is the 6 & 2 option which entitles you to serve in the National Guard for 6 years and then in the Inactive National Guard for 2 years (While in the Inactive National Guard you do not have to attend drills or Annual Training; but you may be called to serve in a National Emergency). The other option is the 4 & 4 tour during which you would serve for 4 years in the National Guard and 4 years in the inactive National Guard. Not all benefits are available for a 4 & 4 enlistment. Note that in order to take advantage of education benefits, members must enlist under the 6 & 2 option.
Veterans who have served in any branch of the military have additional options available to them, including a "Try One" program which allows a veteran to serve for only one year on a trial basis before committing to a full enlistment.