Some people find feet sexually attractive. It doesn't have anything to do with the gender. Guys can like feet and girls can like em.
There's nothing wrong with it either, everyone has their own preferrences.
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Ask her for her preferences. Does she like massages? If so then gently massage her feet and toes until you can reach a point where kissing or licking might be acceptable. Make sure you ask her if it feels good when you kiss or lick her feet before proceeding.
It depends on the girl. I am a kindhearted and musical girl and I don't like my feet worshiped. On the other hand, my BFF is a little miss drama queen and she likes it when boys kiss the ground she walks on (metaphorically, of course). Sorry if this is unclear but that is the best explanation I can give at the moment.