Hi
@greensquares and welcome to the forums.
An hba1c of 70 corresponds to an average blood sugar of 11mmol/L over the last 3 months (or 198mg/dL if you are using US units). Non diabetics normally stay between 4 and 8 mmol/L so you can see that that reading makes you well and truly diabetic.

I am assuming that your doctor says you are T2 diabetic? (Most likely at your age but it's not impossible for you to be slow onset T1).
As a T2, you don't process carbs well, you are insulin resistant and produce more and more insulin to attempt to use the carbs in your diet. Eventually, your insulin resistance increases to the point where your body can't cope with the carb load and your blood sugar goes up above normal.
The easiest way to reduce your levels is to eat less carbs (imagine if you were gluten intolerant, you'd cut down on bread), and many of the T2s here have returned their levels to normal by doing this.)
@Goonergal has supplied some useful links but I'm going to add this quick start guide by
@JoKalsbeek
JoKalsbeek's blog | Diabetes Forum • The Global Diabetes Community
Your doctor will assume that you won't change your diet and give you an escalating set of drugs culminating in insulin injections after sufficient years. Plenty of people do this but it is a choice. If you aren't wedded to carbs I'd suggest trying to restrict them before going the heavy duty medication route.
Metformin is a mild drug which has a number of benefits other than that of slightly lowering your blood sugar. Some people find it gives them gastric issues but you may well find that it works for you. Particularly if you lower your carbs at the same time.
Good luck.