There are a couple of ways to sell bitcoins for fiat currency (like USD, Euro,etc)

Directly trade with someone who wants to buy bitcoins using currency.

Find an online exchange that would buy your bitcoins for some currency (mostly *USD).

A few exchanges are listed here. These are online market places that buy and sell bitcoins. Most probably you'll need to create an account (these accounts are used to trade on the site, it is not your bank account) on these sites to trade. You can trade your bitcoins for any currency that is available on the site. If you need to convert to a currency that is not listed on the site, convert your bitcoins to a general currency like US Dollar or Euro and then use a normal forex market or bank to exchange it for the currency of your choice.

IMPORTANT: Once you sell your bitcoins, you'll need to withdraw your money from the account on the site to your bank account to be able to use it.