Quick answer
A спра́вка is a certificate confirming something. Russian bureaucracy runs on them, and you will be asked for several.
Documents
The office
What they say
At the window
Bring copies of everything
The unwritten rule of any Russian office: bring the original, a photocopy of the original, and a copy of the copy if you can. A missing ко́пия is the most common reason to be sent away, and there is often a copy shop next door for exactly that reason.
Registration
Foreign visitors staying more than a few working days must be registered at their address — hotels do it automatically, private hosts have to file it. Keep the little slip: it is the регистра́ция, and it is the document police ask for alongside the passport.
Typical mistakes
мой о́чередь
моя́ о́чередь
о́чередь is feminine.
Мне ну́жно спра́вка.
Мне нужна́ спра́вка.
ну́жен agrees with the thing needed.
Check yourself
What is a спра́вка?