When you begin creating with resin it can be very daunting! AND EXPENSIVE! Not everything is necessary. When I first started I put down a garbage bag under my work and mixed in a red solo cup with a scrap piece of wood! It's ok to start with some basics and add more as you go.
Now that I've been at it awhile it's easy to know what is needed and what's not. There are definitely some must have supplies but there's also some that aren't quite as necessary but still nice to have. The type of art or project you want to do may affect this list  but it should cover the basics for most projects.
Obviously safety is a big one if you're going to be working with resin so I have included safety gear in my list as well as some tools you'll need for after your project is cured so you can finish it off nicely.
To make it easy for you I've put everything into one nice shoppable list that you can find right here: https://benable.com/Bouncingoffthewalls/what-you-need-to-get-started-with-resin
If I've missed anything on this list let me know! I love to find ways to reuse, recycle and make less waste in this craft that can be at times somewhat wasteful.