Find out how OpenGL deals with texture coordinates which fall out of range and how it deals with texels that do not have a 1 to 1 correspondence with pixels.
Understanding how textures work in OpenGL can be confusing. Especially for someone new to OpenGL. I Hope that this brief introduction to OpenGL textures can help you.
Rendering text in OpenGL is not as intuitive as you may think. There are some tricks which you have to apply. Learn how to render Text in mobile devices using OpenGL ES.