Microsoft has just released the source code for the two most popular shaders used in the XNA Framework. Go to the Education Catalog on the Creator’s Club site, and see them for yourself: the SpriteBatch shader and the BasicEffect shader.
This is awesome, as it provides lots of insight on how things work in the background, and also offer a good place to learn from. Beware that these are complex shaders, and you should only delve into them after you have a good understanding of HLSL. Until then, you should use the XNA classes, as they are 🙂