I first got into 3D CG sometime in 2004. I was still in business and commerce, and had some health troubles. I wanted to focus on a hobby which would take my mind off these and other issues. I had zero experience with 3D, but had a decent know-how of 2D digital art. There were many websites offering 3D assets (content) for free or for sale, and 3D graphics had a rising wow factor back then.
My personal life-long interest has been portraiture sketching (yes, with hand and pencil or ink). The free Daz Studio in mid 00s decade, was basically an alpha-stage software. Poser was about the only viable software for photorealistic CG art of humans, and which didn't require me to fork out an arm and a leg, or learn how to model, rig and texture my own figure. For a beginner in 3D CG human art, that was my software solution. The moment they launched a sale campaign, I bought Poser Artist and that was it ! So much content to download free, and so much content available for a price as well. Websites which existed then included those which remain today, but also PoserPros (bought by DAZ3D but lost much content thereafter due to vendor withdrawal), FaerieWylde, FaerieRealms, RuntimeDNA (now absorbed by DAZ3D), to name a few.
There is one important caveat about living in this 3D CG world of pre-created content and other people's visions: those assets get obliterated during a PC hardware crash, and you really need to be at the top of your backup game. After a while, it becomes too unwieldy to keep backing up and restoring your acquired 3D CG assets. And your own vision for a particular 3D CG genre may not be fed by the type and availability of content anyway.
So, my advice is: if you are a complete newbie to 3D CG software and content, certainly go and play with DAZ Studio (free) or Poser (need to buy Poser), familiarise yourself with how 3D CG art is made, and use the out-of-the-box content. After you've understood a little better how it all works, I strongly urge you to move on to learning how to model your own geometries, how to rig your own figures, and how to texture them. Blender 3D is free and very powerful, but its user interface can confuse you, so take it one step at a time. I recommend Blender for newbies because you can use the figures generated by MakeHuman or ManuelBastioniLabs plug-ins, which are also free. You may of course buy other software to learn how to make your own figures. (I am a Shade 3D user. And I am still learning how to create content in both Shade 3d and Blender 3D.)
I am not promoting these other online stores for 3d content, but you may need to know about them: Smith Micro Graphics' Content Paradise, Renderosity, CGBytes, The Fantasies Attic, Fantasies Realm, Vanishing Point, PoserWorld, and several non-English Language ones, eg: Download Market (Japanese). I don't read German, so I can't give you the German web shops, sorry.