if you have no experience with 3D modelling, and want something you can use fast for this jam I suggest MagicaVoxel. it's easy to learn, fast to model something in and you can safe your models for Unity with the colours you used in MagicaVoxel. you only have a 126x126x126 voxel space to work in, looks small in the beginning, but you can make something nice fast, and all your models will look consistent. Keep in mind you can model bigger things like a building in smaller parts and combine them later in your game engine. MagicaVoxel software is free. if you later want to improve you can look into Blender or Maya (both free if you make a student account for Maya).  These 2 both have their pro's and con's, and their armies of people on the internet fighting each other, but at the end of the day, use what you feel comfortable with, this will push you the furthest.

hey, you need to write your own code for the jam, so you cant copy it