You need to know the name of the sheet to apply the select statement at it.
And you need to add the special char $
at the end of the name.
Supposing you have a sheet named MyFirstSheet
then you can select rows from it with
OleDbDataAdapter cmd = new OleDbDataAdapter("select * from [MyFirstSheet$]", con);
In case you don't know the names of your sheets you could call
using (OleDbConnection con = new OleDbConnection(connectionString))
{
con.Open();
DataTable dt = con.GetSchema("Tables");
string firstSheet = dt.Rows[0]["TABLE_NAME"].ToString();
...... work with the first sheet .....
}
This example should give you the name of the first sheet in the excel file (other sheets are available in the successive rows after the first)