How to get data out of sqlite database by using a function?

I am making a application in Xamarin forms but whatever I try I can't get the sqlite database working. I want to select all Categories where menu_ID = 1 how can I do this? I need this code inside a other page (CategoriePage.xaml.cs) can somewann help me with this?

Here is some code that I use:

(Tables.cs):

namespace AmsterdamTheMapV3
{
    public class Categories
    {
        [PrimaryKey, AutoIncrement]
        public int ID { get; set; }
        public int Menu_ID { get; set; }
        public string Name { get; set; }

        public Categories()
        {
        }
    }

    public class Places
    {
        [PrimaryKey, AutoIncrement]
        public int ID { get; set; }
        public int Categorie_ID { get; set; }
        public string Name { get; set; }
        public Boolean Featured { get; set; }
        public string OpenHours { get; set; }
        public string Info { get; set; }
        public string Images { get; set; }
        public string Phone { get; set; }
        public string Website { get; set; }
        public string Adress { get; set; }

        public Places()
        {
        }
    }

    public class Events
    {
        [PrimaryKey, AutoIncrement]
        public int ID { get; set; }

        public Events()
        {
        }
    }
}

DB helper (TheMapDB.cs):

public class TheMapDB
{
    private SQLiteConnection db;

    public TheMapDB()
    {
        //Getting conection and Creating table  
        db = DependencyService.Get<ISQLite>().GetConnection();
        db.CreateTable<Categories>();
        db.CreateTable<Places>();
        db.CreateTable<Events>();

        var categories = new Categories()
        {
            ID = 1,
            Menu_ID = 1,
            Name = "test"
        };
        db.Insert(categories);   // Insert the object in the database
    }


    public IEnumerable<Categories> GetCategories()
    {
        return (from t in db.Table<Categories>() select t).ToList();
    }

    //Get specific Categorie
    public Categories GetCategorie(int id)
    {
        return db.Table<Categories>().FirstOrDefault(t => t.ID == id);
    }

    //Delete specific Categorie
    public void DeleteCategorie(int id)
    {
        db.Delete<Categories>(id);
    }

    //Add new student to Categorie
    public void AddCategorie(Categories categorie)
    {
        db.Insert(categorie);
    }
}
}

CategoriePage.xaml.cs:

 public partial class CategoriePage : ContentPage
    {
        static TheMapDB database;
        TheMapDB categorie = new TheMapDB();

        public CategoriePage(String txt)
        {
            InitializeComponent();
            var layout = new StackLayout { Padding = new Thickness(5, 10) };
            this.Content = layout;
            if(txt.Equals("1"))
            {
                txt = "this text is number 1";
                //needed code can't find soluction

            }
            var label = new Label { Text = txt, TextColor = Color.FromHex("#77d065"), FontSize = 20 };
            layout.Children.Add(label);
        }
    }

thank you in advance,