8-bit Workshop[0] got me started on 2600 programming. I've since moved on to a local setup using Stella (emulator) and DASM (assembler), but it's a really easy place to start. The book advertised there is decent, but there are also some really good tutorials that can be found online, this one[1] is pretty popular.

[0] https://8bitworkshop.com/

[1] https://atariage.com/forums/topic/33233-sorted-table-of-cont...