It is wise to start looking as soon as you know you are coming to CSU, or at least 60-90 days out from when you hope to arrive in Fort Collins, whether you plan to live on or off campus. Vacancy rates near campus tend to be fairly low and are seized early.
University apartments are available for students, staff, and faculty who are affiliated with CSU. University Housing provides housing for committed couples, couples with children, single parents, and single graduate or undergraduate students 23 or older. As a graduate student, you must be registered for a minimum of one credit hour or for continuous registration if you are working on research, a thesis, or dissertation. A letter from your advisor may be required for verification. You do not have to be registered for summer classes, but must register for the following fall semester in order to maintain your apartment status over the summer months.
Faculty and staff residents must be full-time employees, post-doctoral fellows, or visiting faculty. Faculty and staff are assigned to apartments after all students have been placed.
There are also many housing options available off campus.
The university maintains a site listing available rentals at: http://www.ocssral.colostate.edu/1off-campus-housing
There are also several rental housing sites that you can use including Craigslist and http://northerncoloradorentals.com/