Hi Spencercat, welcome to the forum! We all started where you are, and know what you're going through.
Very similar with my kitty, she is 16.5 and was diagnosed just before Thanksgiving. She also had a UTI.
Did your vet start you on insulin? Do you know what his blood glucose levels were? What diet are you feeding currently? There are some great recommendations for food that are not very expensive. Many of us feed Fancy Feast Classics, which you can buy at any grocery or pet store. With kidney issues you need to be a little more careful, but there are still plenty of good options.
Diet change an clearing up the UTI can have a significant impact on blood glucose levels.
There's tons of information around here so read around, but these forums are also full of friendly people so don't be afraid to ask anything and everything. Some of the long-timers can help get you on track!