Hello GAMELMPR,
Let's go to the answers for your questions:
Question # 01
Answer: Sorry I do not know any manual or tutorial in this forum that teaches how to make custom textures perhaps someone else might know a good one.
Furthermore, in C: \ Program Files \ The Game Creators \ FPS Creator \ docs folder you can find a pdf file called Official Community Guide to FPS Creator compiled by Nickdide with some helpful tutorials about textures.
With respect to textures, you can do them if you know how to or buy them from third parties.
After making your textures you can use a handy program called Firm V2 by DarkFoil to customize things such as walls, floors, and so on. Here is the download link:
http://fps-files.com/download.php?view.132
Question # 02
Answer: Make 3D models or meshes (entities or segments and so on) you will need a 3D modeling software such as the famous 3DS Max or Maya (very expensive but impressive pieces of software, very powerful both), Lightwave 3D, Blender, Skechup, Milkshape, Fragmotion, just to name a few of them.
Question # 03
Answer: UV mapping can be explained as the process of making 3D modeling to 2D image representation of a 3D model.
You can learn more about this on the link below:
http://en.wikipedia.org/wiki/UV_mapping
I hope this helps to clarify a bit your questions.
Have fun!
Northern