Well... I wouldent say I know what game desighn is all about BUT, I have had some experiences that might be usefull...
As far as "where to start" I think its best to work through some tutes, mess around abit and just kinda get your bearings. Then (if your planning on making a game) its always best to think simple, always come up with ideas and then think to yourself "can i realistically do this in RF?" then if your not sure you can ask for ideas or just "can this be done" on the forums... After that well just before you start your project, try and make a basic outline of your game (write this stuff down) eg, story, characters, levels, weapons, pickups, all the basics, ive attached a basic blank desighn document which you can then try and fill in with yourown ideas. This is important because it helps you to keep a focus and will help you work out what you need to do to get where your going (a finished game or demo or whatever)
becore you start your levels, always plan them out first in your head and then either in a graphic desighn app or good old pen and paper, also do some basic FPS practice levels to see what does does not work, by that I mean just using the stuff that comes with RF make a few custom levels, just to get some practise
now onto texturing, I found that with my first game since it was based on a real world location i could do a lot of photography and then edit the images to be seamless, in the free "texture maker" (search for it either in the forums or on google, i heard about it here but can remember the link) there are also some great texture resources on the web and the amazing photorealistic texturepack that you can get off the RF download site
Now when it comes to sound, you might want to try and make your own sounds, (with a recorder etc.) you will also want to download Audacity or some other sound editor as it will come in handy down the line... You might try the web for sounds or mabey buy a CD of free sound effects (I saw one in a tech shop ounce that had 6000 mp3's) and it was only a few quid (£ pound sterling sorry)
So the way I normally work is lay out the game plan, then set to work on making media (like levels models, textures sounds, scripts etc.) then putting it all together both as we go and also finally testing everything and making last minute changes and compiling at the end...
I hope that this was helpfull, if you have any specific questions please ask...
:EDIT: sorry about the desighn doc, but I lost the blank copy that I had, so im afraid that this is the desighn doc for my previous game School of the Dead, but its not hard to change it for another game, I just backspaced it all and used this one for my current game project anyway. ALSO ive included the ORIGINAL DESIGHN DOCUMENT FOR THE FIRST DOOM GAME!!! It was in the same folder and i figured i'd add it as an example of a professional one and also I think that "the doom bible" as its now called is quite a signifigant part of game dev history...
:EDIT: :EDIT: GAR! the file is too big to attach with "the doom bible" so ill attach the basic desighn doc here and post the doom bible in another post... or ill upload it on file front and place the link here...