Cat is lethargic and shaking, urgent?!?

  My 12 year old cat is trembling and breathing heavily, he's not interested in food and didn't even look when I opened a can of tuna. He was fine yesterday, does anybody know what could be wrong?
Call a cab if you don't have a car, or some vets will make housecalls. Heavy breathing could be many many things such as the other poster suggested. He needs to be seen by a vet, call a friend who has a car, do whatever you can to get him there if they can't come to you.
It is URGENT that you take the cat to the vet. There is obviously something seriously wrong with the cat. Just because you dont have a car doesn't mean that you can not take him to a vet. There is a thing called taxi, and a bus. There are also vets that make house calls too out there. Get him the medical care he needs.
Get him to a vet, urgently ! There could be all sorts of reasons for this. He could have a chest infection, he might have swallowed string or something that has knotted in his intestines, he could have cancer or leukemia, the list is endless, but if you don't get him to a vet soon it will be too late.
If this is so urgent you should be getting on the phone with an animal hospital and bringing him in. People these days,