Enrolling in Army ROTC is not, strictly speaking, joining the Army. You will not be sent to boot camp. However, the primary purpose of the Army ROTC program is to produce its Officers, so you must agree to serve as Officers in the Army after graduation to go through the entire program, or if you have received an Army ROTC scholarship. Enrolling in the ROTC Basic Course (the first two years of college) does NOT obligate you to serve unless you have also received a scholarship.
ROTC is the most common way for a college student to receive a commission into the officer corps of the Army. A person cannot just enlist to become an officer. While all individuals who wish to serve their country must meet the same basic educational and physical requirements, officers must also possess a baccalaureate degree, and some preparatory instruction in small unit leadership, planning, and management. College students enrolled in the these Military Science courses assume the rank of Cadet and are trained in the fundamentals of military leadership. Offcers are the leaders of the Army. In addition to leading, they direct and provide vision and structure to the Army.
ROTC is fundamentally a four year progression system, but the Army realizes that there are many circumstances so they have devised many other lateral entry programs. Depending on your particular circumstances, you could feasibly be commissioned in as few as 2 years (4 semesters).
No. However, some individuals with less than three years of school may have to attend the Leadership Training Course in lieu of taking all the basic course requirements. The basic course is essentially the freshman and sophomore military science courses. When a student has completed all of his basic course requirements, or has attended Leadership Training Course and is academically aligned as a Junior, they then become advanced program cadets. The advanced course program is the junior and senior year.
Feel free to view the general Army scholarships available to cadets throughout the country, as well as the ACCP program available to members of the UIC Fire Battalion (of which IIT is a part), here.