Jose,
Take a look here:
MPGuino - EcoModder
You'll find answer to both of your questions. First answer is 7805 voltage stabilizer, second answer is 50k resistor and 5,1V Zener diode. But that's for measuring pulses. For analog signals you need a voltage divider made of two resistors.
And regarding ATTiny there are many versions of the chip and we'll find the right one for the task.