Some links to get us started:
Bit
Byte
Pointer
6502
6502 instruction set
Atari 8-bit documentation online
Atari 8-bit memory map
Atari 800 emulator
Altirra - an Atari 800 emulator for Windows
A ton of Atari 8-bit documentation
Atari 8-bit ROMS
Space Invaders
6502 cross-compiler and tools
Atarimania 8-bit games