If any body sends me Salam or greeting, then how should I reply?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu alaihi wasallam, is His Slave and Messenger.

If a person sends someone Salaams through somebody, it is Sunnah to reply by sending Salaams on both the bearer and the sender. One may reply by saying: "Wa alaika Wa Alaihi al-Salaam" (Peace be upon you and him). Imaam Nawawi said: 'It is likable to reply the bearer also, as narrated in Sunan Nasaai that a person from the tribe of Banu Tameem conveyed the Salaam of his father to the Prophet. The Prophet responded him by saying: "(Peace be upon you and your father". It is also narrated that Khadeejah (May Allaah be pleased with her) replied when the Prophet informed her that Allaah Sends His Salaam to her, she replied: "Allaah is the Salaam and all peace is from Him and peace be upon you and Gabriel". No doubt, he opposed the Sunnah.

Allaah knows best.