Get yourself a Hav-a-Heart Trap. Bait the trap with fresh meat, fish or remove all the food and put the food in the trap. Set the trap according to directions and hide or disguise it. Put it under the woodpile, throw a tarp over it, pile junk on it - whatever. Be sure you can't tell that it's a trap. If you can't tell, the cat probably can't either. Now watch and wait.
It could be several days or only a few minutes. The trap has tripped and you have an animal in there. You don't know if it's a cat unless the cat is yowling. Clear off the camoflage and take the cat, trap and all to the vet or humane society.
If you have taken on an abused or feral cat, my hat is off to you. These cats are very difficult in the beginning, but become the most loving of all cats. They may never be lap-cats or extremely cuddly, but their appreciative love will reward you many times over.
You need to get down to the cat's level, so sit on the ground or a low stool. These cats will be most afraid when you are standing because you are so much larger than they are.
Give it time and speak softly. Name your cat and use that name in a loving tone. Scatter some treats around the enclosure. Your cat will come to associate its name with loving gestures and treats. This is excellent! You want your cat to think of you as something good, not something scary.
Start with treats. Use them to tempt the cat close and as your cat becomes less afraid, offer your hand. Move slowly. Bonding with your feral cat will be very important. Use Fishing Pole Toys at first, then graduate to Tickler Wands. When you feel the time is right, pet the cat behind the ears or stroke it's back. Eventually, the cat will come for scratches behind the ears, base of the tail or under the chin. Offer these freely. Once your cat comes out to greet you when you've been out, you know that your cat is yours and will always love you.
Your cat will also need Balls, a scratching tree that is acceptable and provide all the necessary accoutrements: litter box, bed and perches around the house.You will need a carrier to take your cat to the vet.
Trips to the vet will always be traumatic, and expect your cat to hide for a while on returning home. Give your cat treats and call her name gently and lovingly. Your cat will realize that going to the vet was unfortunate, but that she can generally still trust you. Good luck!