Hello and welcome to Using English.
It's hard to answer such a broad question. Articles are vey, very difficult in English and I personally believe that a natural feeling of when to use a, the, or no article at all is the last barrier to native-level fluency.
However, when it comes to proper nouns, you will always be able to look it up. How does that oranization refer to itself?
In general (and you'll find exceptions), oceans and rivers use "the" (the Pacific, the Nile), lakes do not (except the Great Salt Lake). In general, deserts do (the Gobi, the Saraha, mountains don't (Mount Everest), but mountain chains do (the Alps, the Andes).
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