Welcome to the forum and welcome to film. I'm probably not the best to give advice, as I rarely ask strangers to sit for me, but expect with practice you'll find the right words. As you get portraits made, probably not a bad idea to make a small book of them to have in your camera bag so you can show potential models the kind of portrait you are interested in making. I would avoid making any comments about their looks, just stick with, "I'm hoping to get more practice making portraits." or something along those lines. And I find, if I preface it all with an "you can say no, and I won't be offended" that they hardly ever decline. Be yourself, and approach people with respect as you would in any encounter, and you might find a lot of positive response. Good luck with your photography.