click here for some general information on webcasts and how to go about it.
Yours is a huge question and cannot simply be abswered without knowing the nature of the webcast. I don't necessarily mean the content of the media stream, but its quality and therefore file size is very important, as is selecting an appropriate file format(s) and you will (eventually) have to produce a database to track views/downloads/visitors/archives etc.
You should also give some serious consideration to hearing impaired viewers, since you can't just turn on closed captioning like you would on a TV when watching a program.
You'd need to supply lots of detailed information before a solution could be discussed.
Try running some searches: Yahoo and similar offer webcasting and blogging services and many blogs now have webcasting built in.