It depends on where you go. I try to pick the least damaging items, as low point fast food is just plain hard to do. For instance, a burger place usually has a grilled chicken item. Any grilled chicken sandwich (minus the mayo) will do. Jack in the Box has grilled chicken strips that are super yummy, if you want to avoid a bunch of added carbs ( which of course equal more points). If you don't like chicken, get a burger from the Kid's menu.
If you do Taco Bell, try to get Fresco items. They don't have cheese and added fatty condiments (salsa instead). Pizza is more of a convenient food, than a "fast food," but I think it's worth mentioning. Thin crust cuts points, and avoiding most meats. Ham isn't so bad. I like a thin crust Hawaiian pizza myself (ham and pineapple mmm). But, limit yourself to a couple of pieces. Remember, you can have almost anything on WW in moderation! That's why WW rocks. Weekly points are my savior some weeks, haha. I don't always make the wise choices, and that's what they are for. I hope I helped!