Learn Turkish: Lütfen tekrar söyleyin.

Söyle is the informal or singular command for "say". By adding the suffix -yin (which is the formal -in ending with a buffer y after a vowel), it becomes formal or plural. Since you are using lütfen (please), adding this polite, formal ending is the most natural choice here.

Where is the word for "it" in this sentence?

Turkish frequently drops object pronouns like "it" when the meaning is obvious from context. Just saying tekrar söyleyin (literally "again say") perfectly conveys "say it again" to a Turkish speaker.

Does the word tekrar always go right before the verb?

Usually, yes. In Turkish, words that describe how or when an action happens (adverbs) are typically placed right before the verb they modify. So, tekrar (again) is placed directly before söyleyin (say).

Can söyleyin also mean "tell"?

Yes! The dictionary form söylemek is used for both "to say" and "to tell". If you wanted to say "Please tell me," you would just add the word for "to me" (bana) into the sentence: Lütfen bana söyleyin.