How to draw a triangle
First, double click on this line tool:
Then you will see this in the top right of your screen:
Follow my settings then click on the Shape button.
Then you will see this pop out.
Follow my settings again, then click OK.
Ok now go back to your work area and drag out a Arrowhead,  but don't drag too long until the tail comes out, just the triangle part.

If you need to have a perfect direction, Hold down the Shift key, then drag out a triangle.

Try to find out the differents by pressing Shift key and don't press.
Don't like the size of the triangle? You can change at the Arrowhead Shape setings: