DVLA will ask the GP and / or consultant that you name on your form for comments. If you haven't had an annual review within 12 months, the DVLA will tell you to have a medical. In any case, the doctor writing the report will want to know your blood glucose records over at least a few weeks. If all is OK, you will be issued with a car licence for 3, 2 or 1 years, after which time you will have to go through the same process. If you want to drive a larger vehicle / more seats, there are more stringent requirments. It is an offence not to inform DVLA when you start on insulin.
There's more information, including links to DVLA web pages, in the Driving / DVLA section of this message board, so I'll move this thread there.