Can Someone Help With Employee Rights...?

if you work 40 hour or more at work is that considered full time. Because I work 40 hours or more and they refuse to give me health benefits because technically i'm not listed as ';full time'; but they give me over 40 hours a week. So when i asked them to list me as ';full time'; they said no. And are continuing to give me over 40 hrs even though im listed as ';part time';. What should I do?