You always need decoupling capacitors - 0.1uF ceramic for each power pin. These
are not optional components and if omitted the device may malfunction (or not, or
sometimes work sometimes not, with mysterious and confusing symptoms....)
Factory chips do indeed use 8MHz internal RC oscillator - programming the bootloader
also sets the various fuse bits to specify clock source and brown-out voltage.
The first step is to load a bootloader using an ICSP (perhaps Arduino as ISP) -
The standard ArduinoISP sketch loads the bootloader and Blink sketch, so I'd
recommend adding an LED/resistor to Arduino pin 13 (package pin 17) - then you
can see the download progressing and see the Blink sketch working.