How you do this will depend on a number of factors: since you haven't tried to explain in any detail, I'll assume you know nothing, and just give you the basics.
The easiest way to do this is to read the text file into a single string, and then use the .NET framework to find the match:
Dim text As String = File.ReadAllText("D:\Temp\MyFile.txt")
Dim index As Integer = text.IndexOf("hello")
If index >= 0 Then
There are other ways, but that is pretty much the simplest.