Grass Texture

You use authoring tools like Gimp or Photoshop, or the newly-presented Substance. Bitmap2Material (available at the Asset Store) can take an image you shoot with your camera and make a material out of it, complete with ambient occlusion and height map.

Texturing is a whole field in graphics design, much like animation and rigging and there's a lot of knowledge on the web on how to make textures.

Basically you go out there, take a photo of grass, edit it to make it tileable and set the resulotion to a power of 2 (64x64, 1024x1024 etc.). That, or you search the web for textures. If you do that, however, you have to be careful not to use any protected images. http://www.cgtextures.com/ offer great textures - for a price.

Something to get you started: http://lmgtfy.com/?q=texture+tutorial

thanks a lot, but i mean like 2d Details not as a surface.