Here are a few helpful notes and links for joining these webinars:

The preferred platforms to download and run the webinar client program are Windows PC and Mac. In addition Apps for using an iOS or Android device like a tablet PC are available.

Details on the system requirements can be found in this web page: System Requirements

The webinar offers two different audio options. When joining, the default option is to use your PC microphone & speaker for VoIP, if possible. You may, however, change the audio option to use a telephone. The GUI will then display dial-in number(s), an access code and an audio pin for you to use.

To minimize noise, all attendees are automatically muted when joining. The presenter is able to un-mute individual participants for asking questions. The GUI has a "raise hand" button that allows you to signal the presenter that you have a question or comment.

Find a brief "Attendee Quick Reference Guide" showing the GUI in Attendee Quick Reference Guide (PDF)

Also find audio mode related details and troubleshooting tips at
Audio Fast Help - Attendee
Audio Mode Using VoIP

For more detailed information you may also visit the FAQ pages and the support pages at the provider site. Note, however, that these web pages are not only focused on webinar attendees, but on organizers and presenters as well.
http://www.gotomeeting.com/fec/webinar/webinar_support (FAQ)
http://support.citrixonline.com/GoToWebinar/ (support documents)