My first c# program: Get Data from Sql Server and show it in Textboxes

Question

So I have been programming in VB.net and want to learn C#.

I "successfully" created a simple program that retrieves data from Sql Server during Form1_Load. The program also retrieves data from an MDB file and converts it into a connection string, which I can use throughout the system. I would like to ask for your comments regarding my first time coding in C#.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Data;
using System.Windows.Forms;
using System.Data.SqlClient;
using System.Data.Odbc;
using System.Data.OleDb;



namespace TestInventoryApp
{ 
    public partial class frmForex : Form
    {
        public bool blnFrx = false;
        public bool blnFpass = true;


        public frmForex()
        {
            InitializeComponent();

        }

        static public string connect()
        {
            //I used using because I have read somewhere that 'using' properly disposes an object
            //This method checks if I can connect to sql server using MDBfile
            string strCONNECTIONSTRING;
            string TableName = "BACKUP";
            string constr = @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\MyProject\bin\system_common.mdb";
            string connstr;
            using (OleDbConnection dbCn = new OleDbConnection(constr))
            using (OleDbCommand dbCo = dbCn.CreateCommand())
            {
                dbCo.CommandText = "SELECT m_TableType.TableType AS TableType,m_TableType.DatabaseSystem AS DatabaseSystem,m_TableType.ConnectionString AS ConnectionString FROM m_table,m_TableType WHERE m_table.TableType = m_TableType.TableType AND m_table.TableName = '" + TableName  +"'";
                dbCn.Open();
                using (OleDbDataReader dbRE = dbCo.ExecuteReader())
                {
                    if (dbRE.Read())
                    {
                        //This line of code gets a single string in the MDB file
                        //Then chops it into parts to make a connection string to sql server
                        strCONNECTIONSTRING = dbRE["ConnectionString"].ToString();
                        strCONNECTIONSTRING = strCONNECTIONSTRING.Substring(27);

                        int hhh = strCONNECTIONSTRING.IndexOf(";");
                        int aaa = strCONNECTIONSTRING.IndexOf(";")+ 1;

                        string dtsrc = strCONNECTIONSTRING.Substring(0, hhh);

                        strCONNECTIONSTRING = strCONNECTIONSTRING.Substring(aaa);
                        strCONNECTIONSTRING = strCONNECTIONSTRING.Substring(4);

                        hhh = strCONNECTIONSTRING.IndexOf(";");
                        aaa = strCONNECTIONSTRING.IndexOf(";")+ 1;

                        string userid = strCONNECTIONSTRING.Substring(0, hhh);

                        strCONNECTIONSTRING = strCONNECTIONSTRING.Substring(aaa);
                        strCONNECTIONSTRING = strCONNECTIONSTRING.Substring(4);

                        hhh = strCONNECTIONSTRING.IndexOf(";");
                        aaa = strCONNECTIONSTRING.IndexOf(";") + 1;

                        string password = strCONNECTIONSTRING.Substring(0, hhh);

                        strCONNECTIONSTRING = strCONNECTIONSTRING.Substring(aaa);
                        strCONNECTIONSTRING = strCONNECTIONSTRING.Substring(9);

                        string initcat = strCONNECTIONSTRING;

                        connstr = "Password=" + password + ";Persist Security Info=True;User ID="+ userid +";Initial Catalog=" + initcat + ";Data Source=" + dtsrc + "";

                        try
                        {
                            SqlConnection conn = new SqlConnection(connstr);
                            conn.Open();
                        }
                        // I'm still figuring out how to use messagebox.show(e.exception) in c#
                        catch (Exception e)
                        {
                            return "";

                        }
                        return connstr;
                    }

                }

            }
            // an error occurs everytime, stating that "Not" all code paths return a value, hence this code
            return "";
        }

        public void sp_FRX_getForexToday(string frxDate, string connected)
        {
            //This sub gets data from sql server
            using (SqlConnection conn = new SqlConnection(connected))
            using (SqlCommand sqlcomm = conn.CreateCommand())
            {
                sqlcomm.CommandText = "dbo.sp_FRX_getFRX";
                sqlcomm.CommandType = CommandType.StoredProcedure;
                sqlcomm.Parameters.AddWithValue("@Date", frxDate);

                using (SqlDataAdapter dta = new SqlDataAdapter(sqlcomm))
                using (DataTable dt = new DataTable())
                {
                    dta.Fill(dt);
                    if (dt.Rows.Count > 0)
                    {
                        txtPHP.Text = dt.Rows[0][0].ToString();
                        txtJPY.Text = dt.Rows[0][1].ToString();
                        txtEUR.Text = dt.Rows[0][2].ToString();
                    }
                    else
                    {
                        txtPHP.Text = "";
                        txtJPY.Text = "";
                        txtEUR.Text = "";
                    }

                }

            }


        }


        private void Form1_Load(object sender, EventArgs e)
        {
            string connected;

            connected = connect();

            if (connected.Length > 0)
            {
                MessageBox.Show("Successful");
            }

            blnFpass = false;
            sp_FRX_getForexToday("2019/06/03", connected);

        }

    }
}

Other things that I would like to learn.

1. Regarding method connect, I would like to use the connection throughout the system, so I declared a local variable in Form1_Load named connected and put the connection in it. Is this the best practice? In Vb.Net, I could use conn.open and declare Public conn As New SqlClient.SqlConnection then use conn throughout the code. How can I do it in C#?

Thank you for your time and comments.