Post-undergraduate consideration ideally begins before the end of second year, and in some cases even earlier. This is so you can weave any admission requirements (course or otherwise) into your undergraduate studies. The process begins with a mix of self-reflection, identifying interests to learn what exists in relevant fields, and then researching what type of education is required to pursue that type of career. Carleton's Career Services offers the most comprehensive career advising on campus and an abundance of online resources.
The Science Student Success Centre offers mentorship, online resources, and events pertaining to career planning and networking.
The Undergraduate Administrator can assist with helping determine how your post-undergraduate objectives fit into program requirements.