How can I tell a girl that she has nice feet without creeping her out? And maybe talking her into some foot fetish sex?
You have to just tell her. If that's not her thing, you guys might be able to compromise, but you can't really force someone to like or even experiment with a fetish. :/
I would say after you start getting to know them and you feel comfortable you should bring it up. I don't think it's something that uncommon. As for me I hate feet but if my gf told me she was into that as long as I didn't have to touch her feet we would be okay
Plenty of women would enjoy a foot massage. If that goes well, you can gradually make it sexier. Of course you'll have to explain your interest at some point, but if she likes the way you touch her feet, it's a good start. Many fetish-type sex acts can start with something less fetishy, like if you want to use bondage, you start with just holding someone's wrist during sex. If you start at the extreme end of things, or even use the wrong word, people could overact. Instead, you do things that make the person think, "oh, that feels good". Labeling it can make a difference.