Opthamlgist saw thinning optic nerve in l eye. Didn't know what q's to ask at the time. Progressive? What can I do to avoid glaucoma? Please educate me

Careful monitoring. Cause of thinning is important to know. There are several specific tests for glaucoma, but it is not always obvious in early stages. Eye pressure is the most simple to test but incomplete. To avoid or reduce progressive damage from glaucoma, careful monitoring by an ophthalmologist and treatment if necessary by eyedrops and/or surgery can be effective for controlling eye pressure.
Depends. Glaucoma is a spectrum of pressure higher than the eye can tolerate, thinner corneas, susceptible discs and visual field losses. This is a bit complex to sort out yourself but there is plenty on the internet. You cannot change your anatomy but should be checked frequently and it seems as if you have made step #1. Continue with your ophthalmologist.