If team captain, ask if your team has and preferred bans first of all, and ban those. Then come the basic bans.
If you're in low Divison (Bronze, Silver, Gold), bans usually consists of Amumu, Shen, Blitzcrank, Malphite, and two others, usaully Hecarim/Vi/Darius/Akali).
In higher divisions (Plat+) bans are usually TF, Thresh (more worth than Blitz IMO), Shen, Zed, the newest champion (i.e. currently Lissandra), and usually a top laner (Elise/Zed/Zac).
If you're first pick also you can ask if you want to trade someone, since usually first-picking a mid lane can be hard. Also if a champion that is highly contested is open, ask if anyone wants it.