For Excel 2003 (.xls)

using System.Data.OleDb;
private void button1_Click(object sender, EventArgs e)
{
    try
    {
        OleDbConnection con = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=FileName.xls;Extended Properties='Excel 8.0;HDR=Yes;'");
        OleDbDataAdapter da = new OleDbDataAdapter("select * from [Sheet1$]", con);
        DataSet ds = new DataSet();
        da.Fill(ds);
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
}

For Excel 2007(.xlsx)

using System.Data.OleDb;
private void button1_Click(object sender, EventArgs e)
{
    try
    {
        OleDbConnection con = new OleDbConnection("Provider= Microsoft.ACE.OLEDB.12.0;Data Source=FileName.xlsx; Extended Properties=\"Excel 12.0;HDR=YES;\"");
        OleDbDataAdapter da = new OleDbDataAdapter("select * from [Sheet1$]", con);
        DataSet ds = new DataSet();
        da.Fill(ds);
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
}

Note: Keep Excel file in Debug folder for both cases.