The best resource to learn about Chisel is the online Chisel Bootcamp. This runs in your browser and assumes no prior Scala knowledge. (You may also run this locally via the backing chisel-bootcamp GitHub repository.)
When you're ready to build your own circuits in Chisel, we recommend starting from the Chisel Template repository, which provides a pre-configured project, example design, and testbench. Follow the chisel-template readme to get started.
The following additional resources and references may be useful: