How do I display alpha textures?

I have a material that is yellow and I want to put a texture with some text over the yellow material, the letters are black and the rest of the texture is Alpha.

What is “display” supposed to mean? In the viewport?
Or are we talking about rendering? If so, please at least tell us which render engine you intend to use.

Feed the image into the color slot of a diffuse or other shader. Combine it with the other shader in a mix shader with the image’s alpha output as the factor. (Assuming you’re using the Cycles renderer.)