Updated: Dec 16, 2019
After you get your Game Engine picked and are ready to start building, one of the questions you will probably ask is... Do I have to build everything or can I download content that is ready to be added to my game?
Fortunately there are tons of resources out there to get started building games. Whether you need character sprite sheets, terrain, vehicles, audio or any random objects, there is always a place to download them. The nice part about that is that there is a majority of content out there free, or free to use so long as you give credit to the owner. There are plenty of stuff to purchase as well, and generally the paid items are the best quality. That doesn't mean that what is free is bad though, it just means that you might get more options and more content if it is a paid item.
The 2 main places I go is the GameDevMarket and the Unity Asset Store. Each has free and paid content for both 2D and 3D content. I use Unity for ability to have it already setup for the game engine, but even the items from GameDev doesn't require a lot of effort to use. Another place that I have looked around at is Turbosquid as it has a lot of high quality items, but not everything is free and some tend to cost more. Don't let the price scare you away though, when you look at the content you will realize that they are charging that price for a reason.
You don't need to download everything that is ready to go, sometimes its fun to try and create something. I use a handful or resources to do this, but my favorite is Textures. I especially like this when trying to find seamless textures for a 3D object I am trying to build. Everything on the site is free, but you are limited to 15 downloads a day unless you want to spend a little more to download more at a time. The nice thing about the site is that it is always getting updated with more content. I might not log in for a few weeks and then go back to check something out and find textures I was wanting months ago.
There is a lot of resources out there once you go and search, and it doesn't always cost hundreds of dollars to get. typing "Free game assets" in the search bar will bring up lists of websites to download content. you could even narrow things down to 2D, 3D, audio, etc. if you are looking for something specific.