Open your Paint Brush settings and increase the hardness to 100%, make sure the Spacing is turned on but set to 1%. Then it appears the left side isn't perfectly round and has a slight angle to it something like 80% roundness and 100 degree angle.

At this point your image still won't have thin ends. That's next:
Now here's the challenge, the left side has variations in stroke width -- so you need to go to Shape Dynamics and turn it on then set the Minimum Diameter down to something like 0. You'll notice in the Controls you have to set this, choose Pen Pressure.

Now, the main way to use this is by using a drawing tablet such as a Wacom. If you don't have one then the only option for you is to use the Pen Tool in Photoshop and then right click it and choose Stroke with it set to Simulate Pressure.
