To place a course on pass/fail, you must come to the Registrar's office, fill out the appropriate form, have your adviser sign it, and submit it to the Registrar's office for processing before the deadline. (Click HERE for academic deadlines for on-campus classes and click HERE for academic deadlines for online classes.)
The only classes that can be taken pass/fail are electives.
If the course is being taken to satisfy any requirement (major, major-related, concentration, minor, etc), it cannot be placed on pass/fail.
Freshmen cannot take classes in pass/fail mode.