Buying a first (nice) car

Hi, all my cars have been hand me downs or bought from family.

I have just over 20 grand to spend on a car. I am planning on buying an Octavia VRS estate.

Please could you give me tips on how to approach buying the car from a dealership. What should I look out for? How can I haggle? Should I exchange my current car with the dealership (10 year old Nissan MICRA with 80k on clock) or sell it privately.

Sorry if these are obvious questions - my dad passed this year and I would have asked him.

Thanks