lagoon - honestly, it's not scary. Though I was bloody terrified too. 
So, you need to work out your words-per-minute. For me, a fifty-minute lecture (ie., an hour on the timetable) is about 6000 words.
Then you write down everything you're going to say, down to colloquialisms and so on, exactly as you'd say it. (Speak aloud and then type). Include the bits where you tell them to look at the handout/click through on the powerpoint.
Print it out in double-spaced.
Then you have the text in front of you and you just need to take a breath, go slow, and remember to pause and look at them enough.
It does feel terrifying, but there is a limit to how wrong you can go. You can worry about incorporating fancy bits later.
If you read it over beforehand with someone, you should know if you have the right amount of material, but you can always prepare back-up by asking for questions in the last five minutes then jumping off onto ad-lib in response to that, or by having bullet points at the end that you can raise as questions/ad-lib from.