Being worried about how your presentation will unfold, is only natural, as you would like the presentation to go smoothly. You main concerns could be that you are on the fence on how to start the webinar, or how the sound of your voice should be, or your maybe concerned that something could go wrong entirely in your presentation. What you should do, is be prepared for it, be ready and when you present your next one you will make it even better from the last.
Don't assume that your very first webinar, will go according to plan, because it most likely wont. Also don't assume that the first 10 or so that you do will all be perfect. Something will happen. They could be, the Internet connection will be lost or the Go To Webinar, wont be able to display the screen correctly, or your PowerPoint may not function properly. You may even have an issue with a site that you are trying to display to the audience, and it may not co-operate, this could happen.
Don't just pray that something wont go wrong. What you should do instead is have some sort of plan in case they do.
You need to have alternative, in case a website isn't working and leave that part out. If this happens, you can just present a PowerPoint explaining the majority of your lesson plan, and leave out the part that doesn't work. The best way to solve something is to have something there ready to be used instead, and don't just rely on what you plan on showing the audience.
If something doesn't go according to plan, plan ahead for next time. If you plan to have someone come in and speak, and they didn't show up previously, then properly plan to make sure that they do come next time. If your technology doesn't co-operate with you, rehearse and be ready for next time, and rehearse with your programs and with your equipment. If you had an issue with the way you were speaking in your presentation, prepare and rehearse the speaking part of the presentation, so you get it right the next time.