You can buy any consumer grade coating which are usually easy to apply but for correct application, you have to prep the paint properly
1) wash
2) use iron remover
3) clay if required
4) compound or polish based on paint condition
5) wipe using panel prep
6) apply the coating
Coatings should be applied very carefully to avoid high spots which require some experience.
I suggest you only attempt this if you have some detailing experience or do it under someone's supervision.
This means you not only need coating but other products like iron remover, clay bar or mitt, polish which doesn't contain fillers, panel prep, and coating.
It's highly recommended to stay in a system. Atleast use polish, panel prep, and coating from same manufacturer.