When you open your music score and want to practice this piece, I recommend you first analyze it, by looking at its structure, modulations, cadences, and chords. And the first step in doing all this is finding out the key. Without knowing what the key is, there is a danger of not understanding how the piece is put together.
Sometimes composer writes the key in the title of the piece like "Toccata and Fugue in D Minor". But often the key has to be discovered by the organist. For some people this task is quite complicated. I recorded this video in order to help you understand exactly how to do it.