Q:

Draw a triangle in the real world with OpenGL ES

I am very new in terms of Augmented Reality with some experience on OpenGL. I wanna draw something such as a triangle with OpenGL ES(Android or iOS, doesn't matter for the platform). The background needs to be captured by mobile's camera. Basically the result likes the Pokemon Go, via the camera you got the real world as the background and a Pokemon inserts into the real world.
So, where should I begin?

A:

One approach.

Capture the image and load into a 2D texture map.
Render a quadrilateral with this 2D texture. The quadrilateral will need to be far enough from your virtual camera so that it forms a background (you will want to enable depth testing). The quadrilateral will have to be large enough to cover the background (which depends on both the distance from the camera and your perspective field of view).
Now render your scene (triangles).

The figure below shows the view frustum in the xz-plane once you have things in view coordinates. N and F are the distances to the near and far clipping planes and θ is the vertical field of view -- let a = aspect ratio = w/h of your image (which should match the aspect ratio of your viewport). H is the height of the quad you wish to render and Q is the distance to the quad from the camera. The height of the quad should then be H = 2*Qtan(θ/2). The width of your quad is W = aH.

The distances from the camera of the objects you wish to be in the foreground should be between N and Q.
I am assuming you know how to set of the view matrix (via the "look-at transform") to position of your camera, and set the projection matrix to specify the perspective projection. Also assuming many other things (like how to load a texture map, draw a filled quad with texture coordinates, enable depth testing, etc...). If you need more details let me know.
If you wish to embed the object within the scene that will require some computer vision techniques (ascertaining depth via stereo image pairs). This is a non-trivial problem.

