Triangles are pretty diverse, coming in several flavors. These are scalene triangles, isosceles triangles, right triangles and equilateral triangles. Most of the time you’ll be dealing with one of the first two types of triangles, and these don’t require much more skill than just drawing a straight line. However, the other two types need a different approach in order to draw them reasonably well.
The biggest problem you’ll encounter drawing triangles is getting their lines straight, so just practice your lines, using this guide if you need help.
Scalene and Isosceles Triangles
These are your typical run of the mill triangles. The difference between them is that no two corners of a Scalene triangle are the same, while in an Isosceles triangle two corners are bent at the same angle. A simple method for making them is to place a dot where each corner should go, and then connecting them If one side of your triangle is going to be against something else (like the edge of a rectangle) then don’t draw three new dots. Place only the new third dot and reuse the existing edge.
Right Triangles
Sometimes you’ll want one of the corners of a triangle to be a perfect 90° angle, just like the edge of a square or rectangle. Triangles like this are called “right triangles”, and are simply rectangles that have been divided in half. If this type of triangle is your goal, a useful trick is to actually draw a square, then draw a line connecting two corners and erasing what you didn’t need.
Equilateral Triangles
The third and hardest type of triangle to draw is an equilateral triangle. In this case, all of the angles in the triangle are the same, and all of the sides are the same length. The only foolproof way to draw this type of triangle is using angles. In most cases however, you can fudge it with a little math (pardon my usage of the dirty m word).
How to draw an equilateral triangle without measuring your angles:

First, draw one side of the triangle (the bottom side seems to work the best). 
Measure how long this side is, then multiply that length by 0.83 . This new measurement is how far above the center of your line the last corner is. Place the last dot above the center of the first side.  
Connect the dots to create a nearly perfect equilateral triangle. If you want to be more accurate, use 0.833333333333…. instead of 0.83 . 