The best way to learn how to automate tasks in MS Word or MS Excel is to record a macro that does what you want to achieve, then view the VBA code generated.
Now you and go to your VB2010 code and issue the same commands that the MS Word macro recorder generated for you.
[edit:]
Here is the first part done for you:
Dim TextToFind As String
TextToFind = "String3:"
Selection.Find.ClearFormatting
Selection.Find.Replacement.ClearFormatting
With Selection.Find
.text = TextToFind
.Forward = True
.Wrap = wdFindContinue
.Format = False
.MatchCase = False
.MatchWholeWord = False
.MatchWildcards = False
.MatchSoundsLike = False
.MatchAllWordForms = False
End With
Selection.Find.Execute
Once you have found the marker, you need to find the beginning and end of the text that you want to work with. I have some code that can show you how to move around and select text in a Word Document:
TryParseNumber[
^] function. It won't do exactly what you want but will point you in the right direction.