The usage of bandwidth depends on what kind of features your website will be showing. Do you have a plain text site or do you have videos running or any other apps that are eating up server resources
Bandwidth is important to the success of your website because it determines the speed at which your site connects and uploads. With websites that have nearly instantaneous upload time, people get impatient when having to wait, so they click on another link. The result will be a lost costumer.
You get bandwidth from the web hosting company that you have signed up with. To decide on which company will be the best to meet your websites needs, you will need to research a few.