Learn German: Ruh dich bitte aus.

This is the informal "du" (you) command form. To give a command to one person informally, you take the normal "du" form of the verb (du ruhst) and drop the -st ending and the pronoun du. That leaves you with just ruh.

Why is aus at the end of the sentence?

The full verb here is ausruhen (to rest), which is a separable verb. When you give a command, just like in a normal present tense sentence, the separable prefix (aus) gets chopped off and sent to the very end of the sentence.

Why do we need the word dich here?

In German, the verb for "to rest" is sich ausruhen, which is a reflexive verb. That means you literally say "to rest yourself". Because this command is directed at one person informally ("du"), you use the matching reflexive pronoun dich (yourself).

Can I put bitte at the beginning of the sentence instead?

Yes, you can! Bitte ruh dich aus is perfectly correct. Moving bitte to the middle of the sentence, like in our example, just makes the command sound a little softer and more natural. Notice that the separable prefix aus always stays at the very end.