The number one rule is this: if a word is countable (e.g. one book, two books), you must always use an article (or my, his, etc.):
I read a book. √
I read book.
This is true even if there are adjectives before the noun:
He drives an old car. √
He drives old car.
Never use a or an with a word that is plural (e.g. books, trees) or uncountable (e.g. water, advice):
I asked her for advice. √