When to use "Well" vs "Good"?

Good day and thank you for your question!
These are two words that are commonly mixed up, so it is a good question. 

The main difference between the two is that 'good' is an adjective (it modifies a noun), whereas 'well' is an adverb (it modifies a verb). Here are some examples: 'He did a good job' - 'job' is the noun and 'good' modifies it; 'He did well' - 'did' is the verb and 'well' modifies it. 
So a question you can ask yourself when figuring out which one to use is, 'What am I describing? Am I describing a verb or a noun?' - this should help you to choose correctly. 
A common mistake that people make is by saying 'Very well' instead of 'Very good' - although there is no noun or verb in that phrase, the common agreement is that the word missing would be a noun (job/work). If you want to use 'well' in this context then you have to add the word 'done' (verb) and it will become 'Very well done!'

I hope that these examples help you!