Create a cube. Convert to Mesh. Switch to point edit mode. Create a material, call it red and make it red. Select the front face of the cube and apply the material to it. Create a material, call it blue and make it blue. Select the top face of the cube and apply the material to it.
You now have a cube with 3 materials, the default, red and blue.
Textures are used to make materials. In the simplest form a material is a solid colour. You can also select a png file as a texture for a material property. The most common thing I do is to select an image for the diffuse property of the material. If you have a picture that you want to paste onto the cube face, in materials, click textures, click ... next to diffuse, click "load texture", select your image file, it appears in the list. Click the texture name in the list of textures to select it then click OK. Then repeat the texture selection (you don't need to load it this time because its loaded already) for the ambient property of the material. Apply the material to a cube face and it appears that the image is pasted onto the cube face.
Once you've done above try -
In the UV editor (click the UV button), you can move and scale the image on the side of the cube.
Once you've done above try -
You can save the object as an obj file, and import it into a UV mapping program like Roadkill or UV mapper (@all are there better tools today?) UV mapping allows you to apply a single image to the whole object and precisely control which part of the image is on which face.
Good luck and let me know how you get on....