How can I get the id of the user that is logged in and used it in activities ?

//This is my code in adding news articles

public partial class Articles : System.Web.UI.Page
{
    string cs = ConfigurationManager.ConnectionStrings["NewsDataBaseConnectionString2"].ConnectionString;
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            LoadCategory(); 
        }
    }
    protected void btn_AddArticles_Click(object sender, EventArgs e)
    {

        SqlConnection cn = new SqlConnection(cs);
        cn.Open();
        SqlCommand cm = new SqlCommand("Insert into tbl_AddNews (User_id,title, category_id, details, photo, date) values(@Userid, @Title, @Category, @Details, @photo, @Date)", cn);
        cm.Parameters.AddWithValue("@Title", txtbox_Title.Text);
        cm.Parameters.AddWithValue("@Category", DropDownList1.SelectedValue);
        cm.Parameters.AddWithValue("@Details", txtbox_details.Text);
        string strImg = System.IO.Path.GetFileName(FileUpload1.PostedFile.FileName);
        cm.Parameters.AddWithValue("@photo", strImg);
        cm.Parameters.AddWithValue("@Date", DateTime.Now.ToString());
        cm.ExecuteNonQuery();
        cn.Close();
        FileUpload1.PostedFile.SaveAs(Server.MapPath("Images\\") + strImg);

        
        Response.Redirect("Articles.aspx");
    }

    private void LoadCategory()
    {
        try
        {
            using (SqlConnection con = new SqlConnection(cs))
            {
                SqlCommand cmd = new SqlCommand("select * from tbl_Category", con);
                con.Open();
                DataTable table = new DataTable();
                SqlDataAdapter adapter = new SqlDataAdapter(cmd);
                adapter.Fill(table);

                DropDownList1.DataSource = table;
                DropDownList1.DataValueField = "Category_id";
                DropDownList1.DataTextField = "Category_Name";
                DropDownList1.DataBind();
                DropDownList1.Items.Insert(0, new ListItem("--Select Category--", "0"));
            }
        }

        catch (Exception ex)
        {
            Label1.ForeColor = System.Drawing.Color.Red;
            Label1.Text = "Something went wrong!." + ex.Message + "";
        }
    }
}