You question is confusing. Can you explain better what you want to achive?
I assume you want to draw UI over 3d graphics.
On most devices you cant mix OpenGL context with existing UI controls. You can create your own UI control which contains GL context, but you cant put another system UI control on top of GL context.
So… many developers creates their own UI control library, which is fully rendered using OpenGL calls. Depending on your needs you might need only few controls… like panel, button, textbox, scrollbar, checkbox.
Now… imagine you have two overlapping panels with some content in it on top of 3d content. Your game run in loop and render complete screen in each frame. This mean it render 3d content, switch to 2d mode, render your UI.
To switch to 2d mode you need to setup projection matrix in ortho mode. See glOrtho call ( http://www.khronos.org/opengles/documentation/opengles1_0/html/glOrtho.html ).
For example on IPhone you have screen 320x480. If you want to setup "2d drawing"do this:
glOrtho(0, 320, 0, 480, 0, 1);
After this pass vertex coordinates in 0..320 x 0..480.
Assume content in one of your UI panels doesnt change in every frame and second panel have graphics that changes in every frame.
You can cache content of first panel in some offscreen sufrace like FBO once and then just draw textured quad in panel. On second panel you have to render complete content in every frame.