Ok, I'm new to Modding and I thought I'd start with something simple. So, I would like to change the wall paper in one of the bombed-out shops and maybe play with the mesh of the shop counter. I know I need Blender and can use GIMP for textures, but I have absolutely no idea how to get the textures/meshes out of Fallout, into Blender/GIMP, then back into Fallout. I've looked up tutorials, but most of them are outdated.
Please keep in mind that I am basically illiterate when it comes to technical issues and may even need to be told what button to hit when.
I would really appreciate any help the forums have to offer. Thank you.

The meshes and textures are held in a special 'packed' form, in the .bsa files that you will find in your Fallout\Data directory. You will need to unpack these. FOMM (http://fallout3.nexusmods.com/downloads/file.php?id=640) has a bsa unpacker (accessed under 'tools'). It's best to unpack the meshes and textures to a separate directory rather than within the Fallout directories.

Textures are stored as .dds files, so you will need software to import and export dds files to and from GIMP (links here: http://cs.elderscrolls.com/index.php/DDS_Tools).

For simple texture changes the process is as follows:

1) Import the .dds file to GIMP and edit it.

2) Export as a new .dds.

3) Place your new texture into a new directory within the Fallout\data\textures directory. Be sure to keep your textures (and meshes) separate from the base game files, it will save a lot of trouble.

4) In the GECK, create a new 'texture set' containing a link to your new texture.

5) Still in the GECK, find the mesh that you wish to re-texture. Hit the 'edit button' and you will be presented with a window that includes a section headed 'alternate textures'. Right-click on 'New Texture', and select 'new' : you will then get a list of texture sets, from which you can select the one you just created.
Many models have several different textures, so you might need to play around to re-texture the parts of the model that you are interested in.