Let's take the noun luggage. Is it countable? That is to say, does it have a plural form? No. It is a non-countable noun that describes a category of possible kinds of luggage--bags, suitcases, etc.. To answer the next question--general or specific--we have to look at its use in a sentence.

Example #1: People who travel abroad usually need to buy luggage.    General, so use no article
Example #2: The luggage that John bought for his trip was very expensive.    Specific (notice the modifying clause that John bought for his trip), so use the

Let's try the noun pencil. Is it countable? Yes, objects with a definite shape usually are. Smoke, on the other is non-countable--no definite shape.  But the container  is countable--columns of smoke. Next, we look at the sentence to see if the noun is singular or plural.

Example #3: Pencils are inexpensive.     Plural and general, so no article
Example #4: The pencils that LARC gives away are excellent.    Plural and specific, so use a the

Example #5: Don't go to your exam  without a pencil.   Singular and general, so use an a
Example #6: The pencil that I took to the exam had no eraser.    Singular and specific, use the

One last word of advice. Some nouns are both countable or countable depending on the context in which they are used.

Example #7: The winters that John spent in Montana were usually warm.    A countable use of winter
Example #8: In the Northern Hemisphere, winter is the coldest time of year.    A non-countable use of winter