Very is used before an adjective or adverb in the positive degree. It means ‘to a great extent’.

She is very beautiful.
It was very nice.
He drove very carefully.
He wrote very well.

Much is used before an adjective or adverb in the comparative and superlative degree.

This is much better than that. (NOT This is very better than that.)

Related posts:

1. Formation of the Comparative and the Superlative
2. Correct use of some adjectives