WHEN DO I USE ?
A LOT OF ?
A GREAT DEAL OF?
Much is used with uncountable nouns and many with countable (plural) nouns.
They are often used in questions.
How much water/sugar/money (uncountable nouns) do you want?
How many people/cars/stars (countable nouns) can you see?
You can use a lot of with countable and uncountable nouns.
There are a lot of stars in the sky.
There is a lot of water in the ocean.
This is a simple explanation, but it should give you a basic idea of how to use the words.
I'm not a teacher.