Come up with an idea first. Use simple shapes as placeholders. Tweak the gameplay until it's fun. It's okay to use free asset packs with credit. If you want to learn pixel art, I recommend Brandon James Greer and MortMort. Piskel is a great free program to get started. If you want to learn game design, Let's Talk Game Design, Game Maker's Toolkit, and Design Doc. Good luck in the jam!
The video guide says all assets have to be made from scratch in the time frame of the jam. Any game engine can be used with no restriction as to whether to make a 2d or 3d game.
