You will need streaming server software like freeware red5, commercial Adobe FMS.
As for the bandwidth, it is not much bigger than video files that you transfer. It is easy to see how big say flash video files of a particular bitrate and duration are. You can record files from other web-sites or convert your own files. Estimate the number of simultaneous visitors and you should be able to calculate the bandwidth you will need.
If you don't know how to do it yourself, you may
hire an experienced freelancer.