This is the basic, most common way to convert CAD to USD (or vice versa). You simply log in to your bank account on-line and locate the tool that allows you to select a source account (in CAD) and a destination account (in USD). You then usually have a choice...choose the amount you want to convert, or choose the amount you want to end up with.
If you don't have a USD dollar account, you will need to set one up with your bank or brokerage before you can do this.
This is is the most typical way that people convert currency, and it is usually the most expensive as well. Usually 1.5 to 2.0% of the amount being converted.
USD-CAD Exchange Rate - Bank of Canada
USD-CAD Exchange Rate - Past Five Days (and other tools) - Bank of Canada
Further Related Links and Articles
Action Still Required to Avoid BMO RRSP Currency Conversion Costs - Michael James on Money - October 2011