read data line by line from a textfile

Are you using VB6 or VB.NET?

VB6:  Add a reference to the Microsoft Scripting Runtime Library (scrrun.dll) Project / References.  

Then, code like this:
Dim fso as new FileSystemObject
Dim ts as TextStream
Dim inLine as string

'To Read:
set ts = fso.OpenTextFile("c:\temp\myfile.txt", ForReading)
Do
     inLine = ts.ReadLine
     'Process with your line.
Loop Until ts.AtEndOfStream = True
ts.close

'To Write:
set ts = fso.OpenTextFile("c:\temp\myfile.txt", ForWriting)
ts.WriteLine "Cool!"
ts.Close

Do you need it in VB.NET?

Thanks only in VB6

Here's VB.NET:
Dim inLine as String

'To read
Dim sr As New System.IO.StreamReader("c:\temp\myfile.txt")
Do
     inLine = sr.ReadLine
Loop until sr.EndOfStream = True
sr.Close

'To write
Dim sw as New System.IO.StreamWriter("c:\temp\myfile.txt")
sw.WriteLine("Cool!")
sw.Close()

Thanks can you please tell me how to i read one line from firstfile.txt and write into second file.txt until firstfile.txt is at end.

Dim fso as new FileSystemObject
Dim ts as TextStream
Dim inLine as string

'To Read:
set ts = fso.OpenTextFile("c:\temp\firstfile.txt", ForReading)
Do
     inLine = ts.ReadLine
     'Process with your line.
Loop Until ts.AtEndOfStream = True
ts.close

'To Write:
set ts = fso.OpenTextFile("c:\temp\secondfile.txt", ForWriting)
ts.WriteLine "Cool!"
ts.Close

Don't forget to add the Project Reference, or you won't be able to use the FileSystemObject:

Add a reference to the Microsoft Scripting Runtime Library (scrrun.dll) Project / References.