My cat is very thin what should I do?

My cat is 12 years old and she has always taken great care of herself but latley she's sort of let herself go, it's been down hill for the past 2 years sense the death of another cat. She use to be sort of - well temperamental and now she's very cuddly and that part was nice but now she's thinner and she's loosing her hair. She's has a bunch of fleas but we're waiting for a warmer day before we give her a bath, we've been giving her soft food along with her hard food and we've always given her a bowl of milk each day. I don't know what else to do because I feel so bad for her, we've had her for such a long time and this is what started to happen to the other cat who lived to be 17, she started to loose weight even though she ate as much as always and her hair started to fall out and I just don't want the same thing to happen to this cat, any ideas?