The first mission you'll receive is from your instructor at the academy. Head to the Kage's office which is at the center of the village. Once you reach the office, head upstairs and talk to Midori, the mission giver in Sand and ask for information about missions.
As a ninja of the Sand Vilalge, your job is to take on missions for villagers. The Mission Assignment Desk is located in the Kazekage Office Building L2. You can take up to 3 missions daily, they reset at midnight (EST)
Ask Midori at the Mission Assignment Desk for Information.