Hi Joaquin,
Firstly, just to set your mind at ease, once you get this going it will be 2nd nature. I've found the Arduino-tiny core to be more forgiving (here) than the one you've cited, but your mileage may vary.
Second, there's no need for any external clock unless you absolutely need one - and since that would use 2 of the legs I'd avoid that. So unless you've set up your ATtiny to expect an external clock source the issue is elsewhere.
I may be wrong, but I typically take that error message to mean there's no response coming back from the target chip. There are a few things to check that you've completed.
Do you have the target (set under the Tools / Board menu) set to the ATtiny85 @ 8MHz?
Did you do the step to 'burn the bootloader'? That actually doesn't load a bootloader onto the ATTiny but it does set some important 'fuses' inside the chip which makes it addressable by the Arduino IDE sketch later.
Do you have the programmer (set under the Tools / Programmer menu) set to Arduino as ISP?
Cheers ! Geoff