Learn Swedish: Du är barnet. - You are the child.

In Swedish, you don't usually use a separate word for "the" before the noun. Instead, you attach an ending directly to the noun. Barn means "child", and adding the -et ending gives you barnet, which means "the child".

How do I know whether to add -en or -et to the end of a word to mean "the"?

It depends on the grammatical gender of the noun. Swedish nouns are either en-words or ett-words. Because "a child" is ett barn, it gets the -et ending to become barnet. If it were an en-word like en katt (a cat), it would get an -en ending to become katten (the cat).

Is "du" used for talking to one person or multiple people?

Du is strictly singular, meaning you use it when talking to exactly one person. If you wanted to say "you" to a group of people, you would use the plural pronoun ni instead.

Does the verb "är" change if I want to say "I am" or "he is"?

No, and this is one of the best things about Swedish! Unlike English (I am, you are, he is), Swedish verbs don't change based on the subject pronoun. It is always är: jag är (I am), du är (you are), han är (he is), and so on.