That's why you need some kind of tutorial.
I knew who to create a font at fly and when I tryied to use

RTB.SelectionFont.Bold = True

It gave me an error message that the property was read-only. Samething with other properties of font. But when I look at the property "SelectionFont", it said you can use it to get or set the font is rich text box. So I created the font of fly and changed the whole font.
These "features" really suck in VB .NET though.