Creating a new Level

Level CreateLevel(Autodesk.Revit.DB.Document document)
{
    // The elevation to apply to the new level
    double elevation = 20.0; 

    // Begin to create a level
    Level level = Level.Create(document, elevation);
    if (null == level)
    {
        throw new Exception("Create a new level failed.");
    }

    // Change the level name
    level.Name = "New level";

    return level;
}

ref: http://help.autodesk.com/view/RVT/2017/ENU/?guid=GUID-AA1B1CD4-3988-4F90-9C19-CE0DC1EC2039