A string doesn't have any formatting associated with it - it can't be "bold" or "underlined" or even "Times New Roman" - it's is literally just a sequence of characters.
Formatting is only applied when it is presented to an output device, be that a browser, a form, or a printer - and then the method to apply that formatting will vary from output device to output device. For a webpage, to boldify text would mean adding bold tags round it:
this is normal <b>this is bold</b> this is normal again
For printing, you would need to use Graphics.DrawString three times, with a Bold font in the middle, and that's complicated because you need to measure the strings to make sure you are locating them in the right place on the page when you print them - bold characters are wider than non bold for the same font and size.
For a WinForms app, it's even worse: you can't apply formatting to any part of a string in a TextBox or Label control, but can in a RichTextBox provided you include RTF code in your string.
So no, it's not a "basic" question - and it's very, very far from a trivial solution depending on the environment you are working in.