When I was learning to sew it was a case of buying a pattern and then wandering around the fabric shops, several of them on one town, and feeling the fabric, seeing how it draped , how much it screwed up, what it looked like held up, etc and then choosing something suitable for the pattern.
Now alas, it's a case of buying fabric online, and then seeing what it's like and choose a pattern suitable.
I don't worry overmuch about the fibre content, other than knowing that natural fibres are more comfortable to wear in warm weather. Yes, I do pay some attention to the pattern envelope about how much stretch the fabric needs; it can be more stretchy, but not less. And yes, the stretchiest direction should go around the body.