Well, you probably shouldn't just automatically suck on her toes. One idea you might try is working your way down her body to her toes. This is something that you should definitely talk about with her. Foot fetishes aren't generally seen as the type of fetish that most people have to be secretive about now. For those that have the foot fetish, they may feel like they have to keep it a secret.
You should be honest when you talk with her, but you also need to keep her feelings in mind. You can and should tell her about your foot fetish. Ask her if it would be okay if you took steps to indulge your fetish. Find out what she would be willing to allow you to do to her feet. You could start off by simply rubbing her feet and maybe kissing her feet. Those are good, comfortable ways that you could introduce your foot fetish to your girlfriend. She may be willing to let you do something more overtly sexual with her feet, but you definitely need to ask.