Tranquilo (Tranqui)

"Cálmate"

if it is a command the proper way could be "cálmate" (direct and/or informal) or "calmese" (formal way, still a command) there are many others ways according to the culture and else ;)

All the given suggestions would be valid depending on the context (formal/informal, etc.). One more option, which is very common in Spain, would be 'Relájate'.

En la República Dominicana, decimos tranquilizate. Es muy común.