The first thing is not creating the phonology. It's to decide the conlang use. This will have a huge impact when you need to make decisions about pretty everything in the lang, phonology included.

If it's an auxlang (auxiliar language), make to be easily learned, you would stick in the most common phonemes around the world, and just a few - 20 phonemes is the ideal. Simplest grammar, best grammar. Vocabulary must be extensive, since it's a lang to be used.

If it's an art[istic] lang, just to a fantasy universe, you're free to make a complex phonology and grammar, but you can make a small vocabulary - just one thousand words, in most cases. One adds the vocabulary as would need, and the fantasy world environment impact in the subtle lexical variation. I think it's better make the story before the lang, however some (like Tolkien) made the other way.

If it's a philosophical language, just borrow Latin or Japanese (or anyone you like) phonology and pay special attention to the grammar vocabulary - specially definitions.