Welcome to the FDMB!
Most of us feed our cats Fancy Feast Classics or Friskies pates because they're under 10% carbs, easy to find and affordable. There's a
Food Chart that offers more options too. No kibble or "prescription" food!
Hopefully the insulin your vet ordered was ProZinc but a lot of vets are still using Vetsulin which is fine for dogs but doesn't usually work too well in cats. ProZinc is a veterinary insulin specifically for cats.
The important thing for now is to get Socks onto a low carb canned or raw diet. As long as there weren't any ketones in the picture (your vet would have told you), changing the food first before starting insulin is the best (and safest) time to do it. Lowering the carbs in the food can drastically reduce the blood glucose numbers and some lucky cats can even be diet controlled right away (although most need at least a short time on insulin)
While you're changing foods, it's a good time to learn to home test. Most vets don't discuss home testing with their clients but if you had a diabetic child, you'd be told to test before every shot and it should be the same for our furkids.
Also testing that's done at the vet's office is unreliable due to vet stress which can raise the glucose as much as 200 points. By testing at home, you get more accurate results. By testing before every shot, you make sure it's safe to give insulin at all. By testing in between shots, you learn how well they are really responding to treatment.
Most of us use the Relion Prime meter from WalMart because it's affordable to use. There are "pet" meters, but the strips for them are about $1.00 EACH....the Prime's strips are $17.88/100.
In this thread there are a
couple of video's to help show you how to home test but there are several others on YouTube to watch too.
Again, welcome to the best place you never wanted to be! Keep asking questions! The people here are very generous with their time and knowledge!