Not a Dr. but I do study this stuff since I have family members affected by overweight/diabetes/cardiovascular disease. To understand any of it you have to get a bit science focused.
Clouding the issue is: how did the sugar get there? also, how does it get removed/used?
Primarily there are two ways to get sugar into the blood & make the levels elevated in normal healthly people: [1] either through dietary consumption (as in you ate something that turned into glucose(blood sugar)), or [2] gluconeogenesis (glue-co-knee-oh-genesis, meaning that the body (the liver) produces glucose which is normal, but in some Type 2 Diabetics case, it makes too much glucose.
How does it get used? Glucose is used as fuel for all our organs & cells. Insulin (yes, that stuff that diabetics inject) is normal created by the beta cells of the pancreas and its purpose is to shuttle glucose to the cells so that the cells are fueled. In some people, they cells don't want to follow insulin's advice and use the glucose. This is overly simplified but is called insulin resistance. The body becomes more and more resistant to insulin doing it's job and the blood sugar is allowed to increase.
So what does metformin do to help? This is summarized from the Wikipedia article. Metformin decreases hepatic glucose output (meaning gluconeogenesis by the liver) and metformin increases insulin sensitivity (fights against the resistance) through a couple different ways and and decreases absorption of glucose from the gastrointestinal tract.
So thats it. But wait, metformin attempts to fix methods [1] and methods [2], from my second paragraph. What can you do on your own at no cost and is not a drug to attempt to fix [1] and [2].
You can exercise regularly. It makes your muscles & cells less insulin resistant. I would say resistance training lift weights) & some amount of cardio. More weights, less cardio.
You can change your diet. Number [1] up top is "either through dietary consumption (as in you ate something that turned into glucose(blood sugar))" and we need to identify what turns into glucose. And the answer is dietary carbohydrates or carbs. Soda, candy, cookies, skim milk, fruits, juice, honey, sugar, bread, pizza, bagels, noodles, potatoes, ketchup, tomatoes, tomato sauce are all carbs. Wait, what about complex carbs (crackers) versus simple carbs (table sugar). Complex carbs get broken down via our enzymes in simple carbs (glucose, fructose, dextrose, galactose, maltcose). So even eating whole wheat bread will still raise your blood glucose level.
So what to do? Cut some or most of it all out. A lower-carb diet or a traditional low-carb diet. This is not abullet proof, works wonders diet, but it will allow easy management of your blood glucose levels and perhaps you won't need metformin after adherence to the diet for 6 to 12 months.
Everything I posted it verifiable and true and accepted by conventional medicine & doctors. I (and plenty of others) believe that