Major Career Change --> Grad School Help?
First, do some research on graduate programs you're interested in, to see what the entrance requirements are. Since students come from different majors, programs want to make sure you have the right background. Anything you don't have, you'll want to take as a course, so it depends on how many holes you have to fix.

Side projects and volunteering opportunity in your area, are great to gain experience and show interest, and ensures your passion is solid. Show you have a good work ethic -- since your GPA is average, find other examples to convince graduate admissions that you work hard. Look at what you've done during college and figure out how show you've developed soft skills (communication, leadership, presentation skills...), and so on.

The overriding theme is you need to convince the admissions representatives that you are as likely to succeed as students who took this as their undergraduate degree and did well. This means you will probably need to get a better foundation, as well as experience in the field to document interest.

You can read other related questions for related advice. For example, check here Is it possible to attend graduate school in pure math after undergrad in CSE?