This question already has an answer here:
In order to see a texture on an object, you first must unwrap the object. In Edit Mode, select the entire object and then press U.
From there, there are many options. For a simple plane, I recommend going to top view and pressing project from view.
If you're unhappy with the size of the texture, go into the image editor, select your entire object in edit mode, and scale it up or down.