We had the exact same issue with our cat, Panda. He is an apartment cat so doesn't get to roam free outside like most cats and therefore doesn't lose weight easily! We started him on some diet pet food which the vet suggested to us - didn't take too long for us to see results! It can be really hard though when they meow for more and you've already given them their portion You have to be tough and remember that you're not starving them, you're loving them!
In regards to the sharp claws, you can buy nail clippers from pet stores which are great and easy to use. I highly suggest getting a laser light and turning all the lights off in your house and you'll watch your cat run around like crazy chasing it! Works so well to get them to exercise.
RE: My cat is overweight 
You have to be tough and remember that you're not starving them, you're loving them!
