All player characters will start as Novice Rank humans, as detailed in the Savage Worlds rules. See below for specific details on Realms of Cthulhu characters.
Also, at the start of the campaign, player characters should not have extensive knowledge of the Mythos, but may have had encounters with the unknown. Player characters are expected to generally be from a professional background, meaning an upstanding member of society. Characters may have a shady past, but this should be behind them at this point.