I would not give it milk, but instead, water. Any meat would be good too. Tuna, chicken, turkey even cooked hamburger. Once you start feeding it, it will be yours. Our usual next move is to trap it, take it to a vet, have it spade/altered and then either: keep it, find it a good home, or re-release it if it is a feral cat. Lots depends on the vet visit. It may be so sick that it needs to be put to sleep or maybe its just plain old hungry. You could also call the local animal shelter - that's your call. Good luck.

P. S. The area where I live has an organization that will loan traps and will fix the cat, test it, give it shots etc. for nearly nothing. You might want to check on something like this in your area.