Learn Turkish: Sen kimsin?

In Turkish, to say "you are [something]", you attach a personal suffix directly to the word. Here, -sin is the "you are" ending for the informal "you". Because the only vowel in kim is i, 4-way vowel harmony rules mean the suffix must also use an i.

Do I have to include sen at the beginning, or can I just say kimsin?

You can just say kimsin. Because the -sin ending already clearly means "you are", the subject pronoun sen is optional. Including sen just adds a bit of emphasis, like asking "Who are you?".

The literal translation is "You who are?". Does Turkish not put question words at the beginning?

That's right! Unlike English, which moves question words to the front, Turkish usually leaves the question word in the exact same spot where the answer will go. If the answer is Sen Ahmet'sin (You are Ahmet), the question simply replaces the name with kim to get Sen kimsin?.

Is it polite to use Sen kimsin? with a stranger?

It is better not to. Sen is the informal "you", used for friends, family, or children. If you are speaking to a stranger, a teacher, or in a formal setting, you should use the formal "you", which is siz. Also, just like in English, asking a direct "Who are you?" can sound a bit rude or confrontational, so it is rarely used as a polite greeting!