1) when did he go missing ? was missing complaint filed? if your dad had gone missing for 7 years then only can he be declared legally dead by court .
2) As per the Section 108 of the Indian Evidence Act, a person is presumed to be dead if he is not heard (of being) alive for more than 7 years.
3) your mother can make application to court . on her application court would declare him to be legally dead as he could not be traced inspite of efforts by police
4) your father share would be inherited by all his legal heirs ie your mother and you . then only after carrying out mutation of name in revenue records should property be sold