So usually if I want to emphasize a word in a sentence (casual conversation not professional writing style) I'll capitalize it. Or maybe bold if that formatting option is available. But the word "I" is already capitalized, and bold formatting often isn't visually distinct enough. I've seen some people use "EYE" as a cheeky way to use a word that has the same pronunciation and also use the typical all-caps format. But I think this is not used enough for its intent to always be immediately recognizable, and might be more likely to cause confusion.
If your medium allows for italics, use italicized text to emphasize a word or phrase: I. This seems to be the most popular convention.
If the medium you're using doesn't have italic fonts, you may be able to emphasize a word or phrase by surrounding the word or phrase with asterisks. This practice was adopted by early users of the Internet, and before that, by Usenet users. The practice was common enough that the creator of Markdown decided to use bounding asterisks to render italic text ("The Rise of Bounding Asterisks in Lieu of Italicization for Styling Text").
However, as that same blog post notes, bounding asterisks also indicate something other than emphasis. They can be used to indicate that a word does not represent the idea that's spelled out, but the action named by the word. So, *cough* should not be read as the word "cough" but as an actual cough.
Gruber sees this use as a form of emphasis. I see it as something different and different enough that it may lead to confusion, if your reader can't decide if you're emphasizing a phrase, or narrating an action. Maybe the context would make it clear.
Or you can avoid typographical conventions that may not be obvious to everyone.
Academic style guides, such as the APA and MLA agree with FumbleFingers; they recommend using words, not typography to emphasize ideas