Fasting blood sugar 15.7, is it serious? What treatment is needed?

Fasting blood sugar of 15.7 is not very serious, but it is high enough.

At this level, don't deal with it yourself, it has to be dealt with by an endocrinologist. Oral medication won't bring it down right away, you have to use insulin first and then switch to oral medication. Can't handle it on your own. Even doctors who don't see patients on the internet can't point them in the right direction with a word or two. Don't listen and follow blindly. See a doctor.