you may start with microchip PIC10F200 with only 33 instructions. The tool is not free, check this out with your local MCU PIC suppliers. you may requested a free sample of MCU if you buy tool from them.
I suggest you one of Pic16f84 ,ATMEL AVR family or ARM7 complint devices
you can get the tools -assembler , linker & IDE -for free from microchipp website and you can find there much app. notes