Your first move is to get her somewhere safe. Do you have a large dog crate or kennel? That will be perfect for right now. Pine shavings are an ideal bedding, but they usually come in a pretty large bag, unnecessary for only one chicken. Old towels work as a substitute and will need changing every 2-3 days. If you have somewhere like a garage or laundry room to keep her, that would be great; she can be outdoors as well provided that she has shelter from wind and rain and it isn't too cold (~30 degrees Fahrenheit or warmer would be best). Chickens can typically handle cold quite well, but since she is alone it is harder for her to keep warm.
She needs a commercial feed for now. Find your closest feed store, and ask for a grower feed (or, if they have no grower, a non-medicated chick start). If she is an adult she would typically be on a layer feed, but because she was on her own for some amount of time she can probably use some time on the extra protein of a grower. A grower should be around 18-20% protein, ~2% calcium. Most feed stores will offer some small bags of feed, usually around 5-10 pounds.
She can have most any scrap you want to feed her - but limit grains such as seed and corn to 10% of the diet, and greens about 25%. Don't feed raw potato, avocado, onions, very fat or sugary things, or anything with caffeine. Good luck!
