Here's some sample code, is VB but is easily convertible to C#:
You need to add a reference to Microsoft.Office.Interop.Excel.
<code>Dim oXL As Microsoft.Office.Interop.Excel.Application
Dim oBook As Microsoft.Office.Interop.Excel.Workbook
Dim oSheet As Microsoft.Office.Interop.Excel.Worksheet
Dim vValue As Object
oXL = New Microsoft.Office.Interop.Excel.Application
oBook = oXL.Workbooks.Open(OpenFileDialog1.FileName)
oSheet = oBook.Worksheets("Sheet1")
'Here you have cell value, just build a loop to get the data you want
'This will loop through all rows in the first column
For i As Integer = 0 To oSheet.Rows.Count
vValue = vValue + ";" + oSheet.Cells(1, i).Value
Next
myTextBox.Text = vValue</code>