Hello, and welcome to the forum.
Unfortunately, the 'RS08KA8 does not contain a SCI module, the hardware that will support serial communications. While it is possible to simulate SCI operation in firmware, this is a more complex process, and the timiing is critical.
If you have little previous experience with microcontroller programming, I might suggest that you consider using an alternative device that contains a SCI module. Additionally, the use of the RS08 family will add further programming complication, because of some limitations of this "scaled down" family.
I might suggest that you choose to experiment initially with an evaluation board from the '9S08 family. Most of these devices provide a SCI module. Depending on your ultimate requirements, the '9S08QG8 might be a reasonable choice.
Regards,
Mac