You can create different mark thicknesses by adjusting the pressure on your brush. When you apply heavy pressure for thick marks, be sure to load up your brush with plenty of paint. Otherwise, your brush will run out of paint quickly, creating a rough, "dry brush" effect (which can be cool, but that's a topic for another day!). For a 'smooth' thick mark, lay your brush sideways to fully soak up paint from your palette.
This tip is from my Watercolor Bite-Size Tips series.