I'm is the everyday short form of I am. The little mark ' sits where a letter dropped out: the a of am. You use it all day for who you are, how you feel, and what you are doing: I'm a student, I'm tired, I'm coming. In speaking, I'm is the normal shape; I am without the short form sounds careful, or strong, as when you mean it hard: I am not going. Though actually, I'm does not like to stand alone. After “Are you ready?”, a short I am is fine, but I'm. as the whole answer feels unfinished — I'm wants more words after it. You will see I'm in chats, stories, and even newspapers when someone is talking. In very stiff writing some people still write I am, but that is a dress-code choice, not a different meaning.