1) Wash it with Car Shampoo
2) Claybar the whole car
3) Compound is very aggressive use polish instead to get rid of the yellowness and light scratches
4) Never leave a polished surface without any protection you can use wax, sealants, ceramic/glass coating
Ceramic/Glass Coating is an expensive option it can last anywhere from 1 to 5 years depending on the package you use
A good quality wax/sealant can last anywhere from 2 to 5 weeks
You can either do it all yourself by doing some research except for Ceramic/Glass Coating or you can get it done by a professional
Some people might try to convince you to leave the paint without any protection (Sax, Sealants, Ceramic/Glass Coating)Don't listen to them else the paint will fade again after some time