For multicurrency environment, first you must tick "Allow multi currencies" box from Settings -> Configuration -> Accounting -> Accounting & Finance -> Features. Don't forget to give administrator the access for technical features and tick "Multi Currencies" in the Techinical Settings.

To make a transaction for various currency, you have to make several pricelist that represent the currency that you used. These pricelists could be define in customer form or in the transaction document (sales quotation or purchase quotation).

As for bank, usually you have to make different journal for each currency. And for company, you can set the default currency in Settings -> Companies -> Companies