Answer:
Before a student can attend a Chiropractic college he/she must complete at least 3 years of an undergraduate university degree (eg: bachelor of science). Most students at Chiropractic colleges (at least in N. America) have completed their undergraduate degree. Once you complete at least 3 years of an undergraduate degree you must apply for admission to a chiropractic college.
The chiropractic college you choose to apply to would depend on many personal preferences. For example, the time you want to take to complete the program. Chiropractic college can last 3 or 4 years, depending on which country and/or school you choose to attend. Although the World Health Organization has stated that to become a Doctor of Chiropractic a student must attend no less than 4200 hours of schooling in a Chiropractic college, some schools squish this all into 3 years, others spread it over 4 years.
Another example of how personal preferences can affect your descision of what chiropractic college to attend is how do you want to practice chiropractic. Some schools like Canadian Memorial Chiropractic College (and others) are very evidence-based, in that they teach that scientific evidence should dictate what you do as a chiropractor. Alternatively, some schools like Sherman (and others) still teach "principled chiropractic", where you are taught that belief systems should dictate what you do as a chiropractor.