go to the create tab on the left, add a generic cube. (alternatively, if you didn't delete the startup cube, you can just use that.)
to UV map an object (like for example, the inside of this cube,) drag the arrow looking thing in the upper right corner of your 3D view and open a new view window.
next, hover your mouse cursor over the new 3D view window, and press shift+F10.
look for the box at the bottom left(ish) that says "object mode." you can click this for a couple of other mode options to pop up. while the generic cube object is selected, switch to "edit mode."
select every vertice by pressing A. (you can also use "B" to use a border select tool, or "C", to use a circle select tool.)
While all the vertices are selected, press "U". this opens the UV mapping menu. there are a series of options to UV map that you can experiment with later. (for more complex models, you can even select lines on the model and press ctrl+E and select "mark seam" to create a clear line that will be considered when unwrapping the model, but you can experiment or look that up later. i might include more information on that if i ever get to making actual non-flat models in the tutorial lol) since we want each individual side unwrapped, you want to click "smart UV project."
Now, you have each individual side of the cube unwrapped. You can export this UV template as an image file to work with in an external art program, by moving the mouse over the "UVs" button at the bottom of the UV window, and selecting "Export UV layout."
when working with textures and UV mapping in blender, you probably want to change the object view to "texture", so you can preview the UV mapping in the view. You can also open and close another menu where you can access settings for the project view, by pressing the "N" key. I usually check "only render" in display, and "shadeless" under shading when testing the UV images, so I can see any mistakes. you're going to want to set it to shadeless, because we're going to be looking inside of it and don't want to be in complete shadow when testing the UV mapping.
for this box, i'm going to make a simple room interior. in the unwrapped UV map image, the bottom right square seems to be the floor.
once you've actually drawn something to use as the floor of the scene in your art program of choice, (it can be grass, or an interior floor,) click the open button with the folder icon, to load the image as the UV texture.
if you set the object to be viewed as textured, you should now be seeing your texture on the cube. now zoom to go inside of it and see what it looks like!