With few exceptions, one must be an enlisted member with several years of experience, recommended by their commander, and pass a selection board to become a warrant officer. The Air Force is the only service which does not have warrant officers. The Air Force eliminated their warrant officer positions when Congress created the grades of E-8 and E-9 in the late 60s. The other services elected to retain the warrant ranks, and shifted the emphasis from a promotion process for E-7s to a highly selective system for highly-skilled technicians. There are five separate warrant ranks. Warrant Officers outrank all enlisted members.
Warrant officers are not required to have college degrees (they are selected primarily based upon technical skills and experience), but many of them do.
Commissioned Officers. Commissioned Officers are the "top brass." Their primary function is to provide overall management and leadership in their area of responsibility. Unlike enlisted members and warrant officers, commissioned officers do not specialize as much (with certain exceptions such as pilots, doctors, nurses, and lawyers). Let's take for example, an infantry officer. An enlisted member in the Infantry Branch will have a specific infantry specialty, such as infantryman (MOS 11B), or indirect fire infantryman (11C). Unless that enlisted member retrains, he will remain an 11B or 11C for his career. The officer, however, is designated to the "Infantry Branch." He can start his career in charge of a light infantry platoon, then may move on to be in charge of a mortar platoon, then later in his career he may move on to become a company commander, commanding various types of infantry troops. As he moves up the ranks, he gets more and more experience in the different areas of his branch, and is responsible for commanding more and more troops. All of this has the primary purpose of (ultimately) generating an experienced officer who can command an entire infantry battalion or division.
Commissioned Officers must have a minimum of a four-year bachelor's degree. As they move up the ranks, if they want to get promoted, they will have to earn a masters degree. Commissioned Officers are commissioned through specific commissioning programs, such as one of the military academies (West Point, Naval Academy, Air Force Academy, Coast Guard Academy), ROTC (Reserve Officer Training Corps, or OCS (Officer Candidate School), called OTS (Officer Training School) for the Air Force.
There are ten commissioned officer grades, ranging from the "lowly 2nd Lieutenant" (or Ensign for the Navy/Coast Guard) to the four-star general (or Admiral in the Navy/Coast Guard). Commissioned officers outrank all warrant officers and enlisted personnel.
There are also two basic "types" of commissioned officers: Line and Non-Line. A Non-line officer is a non-combat specialist which includes medical officers (doctors and nurses), lawyers, and chaplains. Non-Line officers cannot command combat troops. For example, let's assume there was an infantry unit in combat, commanded by an infantry lieutenant. A captain, who is a Military Chaplain is attached to the unit. The Captain cannot issue any commands relating to the combat operation to the lieutenant or anyone else in the unit. If the lieutenant dies, command shifts to the highest ranking warrant officer or enlisted member, not to the chaplain captain. The following is not an exact anology, as it's not possible to accurately compare the military to a civilian company or corporation. However, it may help the layman to visualize the differences between enlisted, warrant officers, and commissioned officers.