I am not an artist, I have rarely used Photoshop, and I have never played a Forza game before. I'm really interested in this installment and the really like the designs I'm seeing. But, is it even worth me trying? I mean, some of these designs are damn near photo realistic. How did they get so good? Would I have to dedicate months to learning that stuff? Should I even try?

I wouldn't bother making something photo realistic, but anyone can do simple pixel artwork or copy some basic racecar designs, that's what I did, and I made a good deal of in-game credits that way (in Forza 2).
 
As far as the photoreal stuff, there is an artistic requirement, but it's more patience IMO.  You have some JPG or something you want to put in the game?  You'll have to map it out with grids and take the time to get every little dot and detail in the right place.  It's not worth it for most people.

You have to have a whole lot of patience and some artistic talent for the photo realistic stuff.

u need to have patience when doing it. it is very hard to do. i barely could make kenny or wind waker link in forza 2. it took me a while to do it.  if u really want to do then get a image and pretty much draw on it to make it very basic shape. just a tip u can change the shape of a shape to be one pixel.

" I wouldn't bother making something photo realistic, but anyone can do simple pixel artwork or copy some basic racecar designs, that's what I did, and I made a good deal of in-game credits that way (in Forza 2).  As far as the photoreal stuff, there is an artistic requirement, but it's more patience IMO.  You have some JPG or something you want to put in the game?  You'll have to map it out with grids and take the time to get every little dot and detail in the right place.  It's not worth it for most people. "

I gave up on the photo realistic stuff about 20 minutes after I started. Try making just some basic designs like this. I think they look better on cars then those photo realistic designs do anyways. Lately I've been making video game sprites on my cars. They're pretty easy to make and they look awesome when your done.

Yeah....  just start with anything simple.  Once you get into faces, anime characters, or very complex pictures you have to be really really good with the tools.  being limited to a bunch of shapes means you have to get creative and know how to add more layers and gradients to get that precise look you need. 

You should try to replicate the image you want to do. And you should do it so that you put a grid on top of your image. You can easily do so if you have photoshop for instance. And after that try to replicate every frame you have of the image. This is the easiest way to do some more realistic stuff...

It all depends on the designs you're trying to do. So far i've done my two designs from scratch having never used a Forza editor before (you can check out my storefront @ Sin4profit  only two designs i've made so far).
It takes a lot of practice to figure out how the system works and odd little workarounds here and there ( like today i just found out how to mirror a grouped vinyl set and use it back in editing mode.)
 
Once you get use to the system it should be easy enough (though maybe time consuming, especially as yer first learning) to duplicate designs by adding a grid overlay to the image you want to duplicate. Also, with photorealistic pictures you'll most likely want to posterize the image and then use the grid system.
 
You can do all this with  Paint.net  and get the add on for it called  Grid Maker 2.0  It's all free stuff.

It's worth trying, I have a few really basic designs to attempt when I finally get around to this option.