What have you tried so far? One solution is to read your text file into a string array, add your text to the first array element and then write the string array back out.
Dim lines(65535) As String
szLines = File.ReadAllLines(sourceFile)
will read the file 'sourceFile' into the array 'lines'
Once the 'lines' array has been populated you can't change it, so you will need to copy the string array into another one which you can change (which is where you will add your text), and then write that array to a file.
Hope that helps!