Orchid Growing 101, Lesson 1: The first thing you must understand about orchids is they, unlike terrestrial plants, do much of their respiratory gas exchange through their roots, rather than leaves.
That means that the potting medium must be able to retain moisture, but cannot block the airflow pathways to the roots.
Lesson 2: When we water a plant, most pours right through. Some is absorbed immediately by the plant's roots and the potting medium, and some is held in-between the particles by surface tension. If the potting medium is too fine, too compressed, or too decomposed, those spaces are small and plentiful, so hold enough water completely block those "breathing" pathways for the roots, so they suffocate, die and rot. That, by the way, is the origin of the myth that "orchids must dry out between waterings". It's not the plant that must dry out, its the lousy potting medium! If allowed to dry out, those air flow pathways reopen, allowing the roots to breathe.
A better choice is a coarser potting medium; one in which the spaces between the particles are too big for the water to completely bridge them. That will mean you need to water more often though, so you need to consider your growing conditions and how often you can/want to water when you choose a potting medium.
I hope that helps you a bit getting going.
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