Some article use is idiomatic and has to be learned case-by-case. For example, we say:
- I'm learning to play the piano. (definite article)
- I had an egg for breakfast. (zero article)
In German conversely it's:
- Ich lerne Klavier spielen. (zero article)
- Ich aß ein Ei zum Frühstück. (zum = zu dem: dem = definite article)
There are nonetheless some useful guidelines that apply in most cases where a noun is used and hence a decision is to be made. Basically, this presupposes that you are able to determine if the noun is a count or uncount noun and whether or not you are referring to a specific (or known) instance of that noun. You will then be able to correctly choose the correct article in noun phrases such as in the following sentences such:
- There was an explosion at the station yesterday.
- There was an explosion at a station in the city center yesterday.
- Does he still go to school?
- Did his father go to the school yesterday.
- Is there a school near here?
A more detailed discussion would take up too much space, but if you are in doubt you are recommended to type the 3 basic possibilties for your phrase (play piano, play the piano, play a piano) into a search engine and analyse the results to see which choice best matches your own intended meaning.