I lied to my girlfriend about not being a virgin but I don't know how to tell her now that I am?

Be honest. Why did you lye in the first place? Maybe to impress her by assuming that women like nonvirgin guys more than virgins. If that was your assumption, it is not always true. You have to be honest and truthful to your partner about your sex life.
OKAY TO BE VIRGIN. I wonder why you lied. Nothing wrong with being a virgin. Be honest and decide if now is the right time. Have safe sex if you do get sexually active.