Recently did a webair which involved me using some great sites and apps. Eventbrite is great for organising your webair or any event. It is like your own conference secretary. You can arrange free or pay events very easily and it will keep your particpants informed of where, when how.
There are a number of video conference providers. A good free one but only allows 5 particpants at a time is WebEx . This allows you to give a presentation, have messages sent to you while presenting which is useful for video conferencing and allow you to share a whiteboard. GotoMeeting is another one but is only free for 30 days and then yiu can either pay monthly or annually and its rates are nased on thet number of participants. If anyone knows of a tool that is either free or low cost I would be interested to hear from you! Goto Meeting has the above but also you can speak with your participants in realtime. A very relaible and easy to use tool which is important for successful video conferencing.
In running a webair it is difficult to predict if people who said they would join you will join you! I found Facebook useful for inviting people who are listed online who might be interested. Using facebooks messenger you can copy and paste your live feed to them!