// Top of your script
new 
    Mission [ MAX_PLAYERS ];

// OnPlayerConnect / Disconnect
Mission [ playerid ] = 0;

// Bottom of script
// NOTE: I use ZCMD

COMMAND:startmission ( playerid, params [ ] )
{
    if ( Mission [ playerid ] == 0 )
    {
        if ( IsPlayerInVehicle ( playerid, * ) ) // * = The vehicle ID
        {
            Mission [ playerid ] = 1;
            SetPlayerCheckpoint ( playerid, x, y, z );
        }
        else 
        {
             // message error like " you are not in a truck
        }
   }
   else
   {
        // message error like " you are already in a mission
   }
}

// OnPlayerEnterCheckpoint
if ( Mission [ playerid ] == 1 )
{
    // REWARD
    Mission [ playerid ] = 0;
    return 1;
}