Newbie here, Have not gave my car (98 civic hb) a tune up since 48,000 kms.(only fluids)Now it has 120,000 kms. What parts should I start replacing? eg: spark plugs...

 

If you look in your manual, it will have a section dedicated to Maintaince, It pretty much tells you, depending on what your driving style is, when to do what.

If your not having any problems to date, you are right on track. Keep the oil changed, and the air filter clean, or changed.

In about15-20k it will be time to look into the timing belt, water pump, and auxiliary belts. You might also want to throw some plugs and wires on. Other then that, you don't have to worry about much.