Three questions, and you get the thread, the size, the needle, the point and the stitch length — with the reasoning for each, so you can judge whether it fits the job in front of you.
What are you sewing?
This decides the needle point and, less obviously, the stitch length.
Where will the finished piece live?
Ultraviolet light is what decides nylon or polyester, and it is the choice people most often get wrong.
What kind of seam?
A seam that is meant to be seen is specified differently from one that holds the piece together.
The one thing worth knowing before you start
Stitch length works in opposite directions depending on what you are sewing, and a great deal of advice ignores it.
On woven cloth, more stitches make a stronger seam. The thread passes between the yarns rather than cutting them, and each stitch adds holding power. A&E put the relationship as seam strength = stitches per inch × thread strength × 1.5.
On leather and vinyl, more stitches make a weaker one. Every stitch is a permanent hole, and the closer together they are the more the material is perforated along a line, until the seam tears the way a stamp does. Longer stitches, fewer holes, stronger seam.
So the instinct to sew finer for neatness is right on cloth and wrong on hide. That is the sort of thing a table cannot tell you, which is why this page asks what you are sewing first.
Fuller detail on both: the thread guide and the needle guide.