Respect her wishes, she's your girlfriend. Treat her with respect, and never under any circumstances take advantage of her or what mindframe she's in cause she's high or drunk. That's the best way to gain the trust of a woman. Talk to her about it, tell her she should try it a few times (because the first time you dont normally get high unless you're lucky).

Tell her it would never hurt to try it a little bit, and show her how safe it is. Make sure she know's the facts and shit before she does it, so she isn't all skeptical. Tell her if she doesn't like it after being stoned, then you'll never ever ask her again to smoke unless she wants to.

Don't get her too fucked up though, cause as a beginner that can be a bad experience.