There are some very interesting tools out there that allow you to make completely free webinars with professional features, including simple and easy screen-sharing. Besides, you don't need to be a pro - the tools are very easy to use. Have a look at these:
- AnyMeeting - great features including video conferencing and screen sharing, for free with up to 200 participants.
- JoinMe - ridiculously simple to use with limited features, up to 250 participants.
- Mikogo - free for 25 participants, screen sharing with up to 10 participants.