Hi Wosup,
Welcome to the forum, first of all! I'm sure you'll find lots of information, advice and support here.
You have what is known as a type 2 diabetes, which is essentially where you become insulin resistant, leading in time to elevated blood glucose levels. The metformin you were prescribed should help alieviate some of the symptoms.
Your health team should, following their own code of conduct, have given you advice on how to deal with your diabetes through dietary changes and exercise, so they have jumped the gun, although that isn't unusual it seems.
The biggest single thing you can do to lower your blood glucose levels is to reduce or eliminate the foods which raise them. Sugar, of course, is an obvious culprit and you will benefit hugely from avoiding it altogether. Starchy foods also have a big impact and cause large increases in blood glucose. These include bread, pasta, potatoes, rice and anything made with refined flour. Some type 2's find that if they restrict these foods sufficiently they can control their bs without medication at all.
All the best, and keep in touch.
fergus