Look at the things you should consider before organizing a webinar:
As soon as a person register for your webinar, send them a thank you mail and a confirmation mail. Sending this is not only good manners, but will also help get a confirmation from the registrant side.
To simply run a webinar, there are options available like GoToWebinar, GatherPlace, Freebinar, WebEx and many more. All you need to do is set a date for the webinar and send invitees a link to a sign-up page.