Note: this tutorial is about drawing triangles as a symbol, not drawing them in general. See this tutorial for that.
Like the Square symbol, a Triangle symbol is just a triangular frame. However, making a triangle nestle in another triangle isn’t quite as simple as when you’re working with squares.
Here’s the solution to making a good triangular frame:

Start out by drawing an equilateral triangle. 
Draw a line going straight down from the top point. The longer the line, the thicker the frame. The place where your line stops is the top point of the inner triangle.  
Now for the tricky part. You need to draw two more lines the same length as the one you just drew. Start these at the other corners and draw them at a roughly 60° angle (assume that the last line was 0°).  
Connect the ends of those lines to finish the design. 