I is the word you use when talking about yourself. When you say “I want pizza for dinner” or “I finished my homework,” you're using I to refer to yourself as the person doing the wanting or the finishing.

The word I is a pronoun, which means it stands in for a noun (in this case, your own name). Instead of saying “Marcus thinks Marcus should go outside,” Marcus would simply say “I think I should go outside.”

In English, I is always capitalized, no matter where it appears in a sentence. This makes it unique: most words are only capitalized at the beginning of sentences, but I stays capitalized even in the middle. “Yesterday I went swimming” and “My friend and I built a fort” both keep the I capitalized.

When you're talking about yourself along with others, I usually comes last as a matter of politeness: “My sister and I” rather than “I and my sister.” You might hear people say “Me and my friend went to the store,” but a more formal and grammatically standard form is “My friend and I went to the store.” A helpful trick: remove the other person and see what sounds right. You'd say “I went to the store,” not “Me went to the store.”