"How can I make a one-sided arrow?"
This is a pretty common question, and there are a few ways to do this.
First, in the Shape Tools drop down menu, there's an arrow tool (Shift+L is the shortcut).
Or, you can also do this:
- Use the pen tool (or the line tool; "L" is the shortcut) to draw a line on the canvas
- Select the vertex you want to change
- Select the three dot menu underneath Stroke in the Properties Panel to open the Advanced Stroke menu
- Choose Line Arrow or Triangle Arrow from the Cap dropdown
On a related note, if you're wondering how to round the corners of a rectangle, head over here.