Welcome to SSS krub Karn,
Congratulation krub you have such a great car that almost everone would dream of however great car needs
and regular maintenance like others.
I will give you some basic idea about how you should look after your gem. (others might have some different approaches)
For maintenance wise
1. Check you oil level regularly and change it every 5000-7000 Km (make sure you use
synthetic oil with recommended grade for Subaru, it should be 10W-50 or 10W-60) everytime you change your oil, make sure oil filter is changed.
2. Check your water level regularly, and everytime you change your water make sure you blend water with radiator coolant.
3. For gear oil and final drive (เฟืองท้าย). my approach is that every secend time I change my engine oil or every 10000-14000 km I would change gear oil and final drive oil.
4. Clean your air filter regularly. for me I do it every month.
5. check your brake pads worn often whether they are worn out or not. if can't see them at the caliper, you might look at brake fluid level too if it is near Min level then you should find new pads. (don't compromise with worn out pads it will never be worth) everytime you change your pads it's better to change brake fluid and make sure you use at least DOT 4 or DOT 5 brake fluid because it better suits your car.
That is just a very basic maintenance guide that you should do it regularly.
For performance wise
If I were you I would do nothing with the car and just keep it clean and well maintenanced because RA-R is quite a nearly perfect car in my opinion (after drove my friend's car few times). just learn how to control it until you can make the most out of its performance and you will be surprised how much this car can do.
Good luck krub