SQL Server :: Programmatically Store Record Into Database?

What I want to do is to store new article and for each article choose a category..

I have 3 tables for example :

1. Article table : ArticleID - int , Article - nvarchar(max) .......

2. Categories table : CategoryID - int , CategoryName - nvarchar(150)

3. Article_Categories : ArticleCategoryID - int , ArticleID - int , CategoryID - int (And I set up relation in this table...)

I can store data into tables 1 and 2 that is working fine.. But I stuck with Table 3.

So far my code looks like this :

protected void Button1_Click(object sender, EventArgs e) //Add new Category (working fine)
{

string connectionString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
string insertSql = "INSERT INTO Categories(catname) VALUES(@catname)";
[code]....