How to save bold text into word

In the same way as you do it manually: by selecting range (single character or word or paragraph or set of characters, words or paragraphs) and pushing Bold command from ribbon.

Have a look here:
How to: Programmatically Define and Select Ranges in Documents[^]
How to: Programmatically Format Text in Documents[^]
Range.Bold[^] property ;)

Note: if you save a document as a text, you'll lose formatting.

Tip: Run MS Word application. When new document is created, type some text, then start recording macro. Select text and bold it. Stop recording. Push ALT+F11 to switch to CodePane.

Here is what i get:

Sub Macro1()
'
' Macro1 Macro
'
'
    Selection.MoveUp Unit:=wdLine, Count:=1, Extend:=wdExtend
    Selection.MoveLeft Unit:=wdCharacter, Count:=7, Extend:=wdExtend
    Selection.Font.Bold = wdToggle
End Sub