increment with last id

Question

increment with last  id , how to get  the counted value of particular field in a variable to perform some function using the counted value.   {

  SqlConnection con = newSqlConnection(@"server=CSE-PC; integrated security=true; Database=Bas");

  con.Open();

  SqlCommand com = newSqlCommand("select top(1)  SUBSTRING(CustomerId,1,0) as Last_Id from devi order by Last_Id desc", con);

  strc00 = com.ExecuteScalar().ToString();

  com.ExecuteNonQuery();

                              here i have gave code like this but it does not work any one please give syntax for this question. 

  if (i == 0)  here I need to increment with last  id   how to get  the counted value in a variable. 

  {

         strc00 = "001";

          }

  else

  {

              int j = i + 1;

  strc00 = "001" + j.ToString();

  }

        comboBox1.Items.Add(strc00);

  i++;

  MessageBox.Show("Id Created");

  con.Close();

  }

   }