Sorry for the rough answer, Jack, it was pretty hot round here yesterday :). What kind of assets do you need? For a 2d game or a 3d game? And what engine are you using? A good starting point is always https://opengameart.org/; other places to find free assets: on itch.io https://itch.io/game-assets/free or the unity store: https://assetstore.unity.com/?free=true&orderBy=1 (Read the copyrights instructions of the assets though; see here for info about licences https://creativecommons.org/licenses/by/3.0/).
For sounds look at: https://freesound.org/
See also: http://www.garagegames.com/community/resources/view/23092
To learn, apart from youtube, I can recommend some Udemy courses ( https://www.udemy.com/ ), but wait for a sale, don't buy if they cost more than 12 dollars ...
Happy developing!