If you are using the triangle shape to illustrate and convey special meaning in your poem, look for objects that have the triangle shape, like a tree or a pyramid. You can either write your poem first and try to fit it into the shape or you can use the shape to help you construct the poem, for instance, the first word may be "A" followed by "word, word", followed by "word, word, word" and so on.
To get a feel for how the triangular poem is to be constructed, draw a standard narrow top, flat bottom-style triangle. You will have less room for words at the top as you have at the bottom. Put your poem into the triangle, placing the words so that if and when you take the triangle shape away, your words are in the shape of the triangle.
Now that you have the idea of the construction of a triangular poem, you can experiment how to set your shape. Try writing a triangular poem with the point of triangle on the left, building from left to right or maybe an upside-down triangle, starting at the flat top and ending with one word at the bottom. All triangles are not equilateral triangles, so you might try an obtuse triangle where one side is greater than 90 degrees.