Querida berta
Dear (noun) = cariño Dear (adjective) = querido To = a
querido amigo
Mi querido.
Querido (for a male). Querida (for a female).
mi cariño/a mi querido/a
Querida Señora Rey.
"Estimada Srta. Estrada"
Bastante bien, cariño.
"Querido diario" is how you would say "dear diary" in Spanish.
If you are saying "I'm working my dear" as something that is going on right now, you could say "Estoy trabajando, cariña." If you are talking about the future, "I'll be working, my dear", then it would be "Voy a trabajar, cariña" or "Trabajaré, cariña" Just remember that the Spanish "progressive" tense only applies to present action.
Estoy loco por ti querida/o :-) (female/male 'you')
You can address a letter Dear Fred and Bertha (or whatever the actual names are). Personally, I often address my letter Dear Folks. That way there won't be any dispute about why I put Fred's name before Bertha's.