how do i do a webcast?

Stream your webcast to users with the proper token (which they paid for). You can use something like VLC for a streaming server.

If you want a full technical workup, it will be near impossible to do in a forum. You'll either have to learn and make some decisions on your own, occasionally asking for specific help, or you can hire a consultant.

If it is a one-time sort of thing, use something like Citrix GoToMeeting.