Learn Italian: Voltati e guarda questo quadro. - Turn around and look at this painting.

Voltarsi is a reflexive verb meaning "to turn oneself around." When you give an informal command (using the tu form), object and reflexive pronouns must be attached directly to the end of the verb. So, volta (turn) + ti (yourself) becomes voltati.

Since guarda is a command for a single person, shouldn't it end in -i like the normal present tense?

Not for -are verbs! In the imperative (command) form, regular -are verbs like guardare actually end in -a for the informal tu form. So while the statement "you look" is tu guardi, the command "look!" is guarda.

Why isn't there a word for "at" after guarda?

In English, you have to "look at" something, but in Italian, the verb guardare already means "to look at" or "to watch." You don't need an extra preposition. You just put the thing you are looking at right after the verb: guarda questo quadro.

Why do we use questo instead of another form of the word "this"?