If she's not immediately interested in the idea of going fishing, try first explaining that you're not sitting on a dock with a bobber just waiting. It's putting a puzzle together, making casts, tricking fish with lures. Try to explain, in detail, the satisfaction you get from catching a bass on an artificial.
If that doesn't work, explain that it's really important to you that she at least comes out a few times to give it a shot. If she won't do that for you, then she's selfish, and you should probably find another girl.
Once you get her out, like others have said, make sure you catch some fish. The first time I took my girlfriend out was in the spring, and I tied on some inline spinners, which were guaranteed to catch at least a couple bass, crappie, pickerel, or perch. It worked, and since then she's become more and more addicted. It's a great thing watching her catch the bug, just like I did when I was a kid, and get better and better.
One thing to consider though, is that if she really likes it, she's gonna want to go all the time. Personally, I like fishing alone sometimes, and often with a friend or my brother, so it's tough telling her I'm going fishing with someone else. However, I think that minor predicament is much better than if she hated fishing.
