You don't need HCI classes as an undergrad as very few schools offer that. But if your grades aren't very high, you could consider taking graduate level courses in design, psychology, and cognitive science online or at a local university. Prove you can handle graduate coursework and that your interest in HCI is sincere by working hard at a few courses. Then start looking into graduate school full time. Many universities offer classes online or in person without being formally accepted. In the meantime, read up on usability research methods and practice by usability testing your own prototypes with friends and family. Spend time observing people to practice field research - shadow people shopping at stores or on the street and take notes on ways you could improve user experience problems you identify.
Visit websites of schools you are interested in and reach out to their admissions department to see what the requirements are.
