If you haven't joined a game jam before, we highly recommend giving My First Game Jam a try each summer and winter, as it is led by members of the gamedev industry and very friendly to newcomers! If you are unsure where to start, their guides are an excellent resource.
In addition, check out the following sites...
Pixel Prospector - http://www.pixelprospector.com/ (guides, public domain assets, and more!)Itch.io Jams Calendar - https://itch.io/jams (a frequently-updated list of every game jam hosted on Itch.io)Open Game Art - https://opengameart.org/ (large online directory of creative commons art assets)Incompetech - https://incompetech.com/ (composer Kevin MacLeod's site, filled with hundreds of free songs)Freesound - https://freesound.org/ (large online database of creative commons sound effects)Free Music Archive - http://freemusicarchive.org/ (a large online directory of bands that post creative commons music)
There are many online resources available to veterans and newbies to gamedev alike!