How can I set attributes (bold, underline, …) for individual text elements?

This is not possible with NoteTab (or any plain text editor for that matter). This feature would require storing special control characters in the document. Since NoteTab is a plain text editor, it only saves documents as text without any control codes. Therefore all formatting information would be lost when you open the document again. If you want to format individual text elements, you will need a more sophisticated editor, like a word processor.