Once you know how to paint you can use any paint thickness you want. But as a beginner you want it to be just right.
If the paint is too wet it will smear and drip. It must be less wet.
If paint is too dry it will scumble. Scumbling is a wonderful technique but not for the beginner.
Thick paint is also called impasto. It creates a wonderful effect but require a great deal of experience to control it.
You want paint that is not wet, dry or thick. It covers the canvas and has a tiny bit of thickness. This thickness will make blending colors the easiest.