One of the most annoying things when you first start out is that you literally have almost nothing to work with, no resources of any kind other than whatever you carry around in your mind. Building a resource bank might not be step one but it's one of the only steps you'll ever take that never ends. Even if you don't like something one of your artists or modelers make, you're going to store it anyway because you never know in the future when you mike find a new use for it. Often old models can easily be reused in a new project even if it needs a little updating and that in itself can save hours of time and money freeing up your budget for other things. This includes program code as well, a sequel is easy enough to make if you already have groundwork done. Not many developers decide to start everything fresh for every game they make. So the question then is how to get one started and how to keep it organized.
I start by separating everything into primary categories. Get your models in one place, sounds in another, textures in another, and animation rigs in another. I keep my programming and scripts in separate folders as well though often in a separate location as I keep most media in one location while things like scripts have their own drive. However you choose to organize them you'll want to also set them up into sub categories. For example I keep my male and female characters apart and also separate from humanoid monsters or creatures namely bipeds. While I obviously keep all other creatures or monsters in their own folder, though even they are getting to a point where they might get split up again into more sections. I make sure to have a naming scheme for everything though so I can tell at a glance what I'm looking at before I have to open it. All of these characters though stay separate from environments, objects, and buildings. Objects also get split up into weapons, environmental objects such as street lamps, mailboxes, etc. Buildings get organized into industrial, residential, business, social, and other types. Vehicles are all on their own as well. A similar setup is also in place for textures.
Though obviously you need content before you can actually organize. Honestly there's so many thousands of sites out there I can't really point to just one and say "this is the best" really you just have to look around for exactly whatever you're looking for at the time. Turbosquid is one fine example of a premium resource site, you pay for what you get and it's usually worth it. Though if you're in need of free items be careful what you pick up as many are only free to use for non-commercial projects and will charge royalty fees. This is true of both models and sounds. It's important not to forget sounds because without them you can't convince players your environment, your weapons, or anything else is real and it'll show. There are giant sound libraries though that will probably have anything you can imagine for surprisingly small fees.
All this having been said the best option for building your own library is just to make the content yourself. For that a lot of times you'll need blueprints and those are not only easy to find but often you don't have to worry about fees either. When it comes to making your own sounds you'll need a synthesizer of some kind. Depending on what you have to spend I can suggest fruity loops aka FLStudio, Revision, or pro tools as great places to start with lots of resources. To help get your cities and characters up and running there's tons of max scripts out there for procedural generation or animations that are only a google search away. As always I hope this helps somebody starting out, because that's when you really need help the most.