I teach a semester-long survey course in the history of science every few years. A few very basic tips:

• You can't teach all of it, not very well, if you're only teaching 10 classes or so (or even 20). So you have to decide what you find most interesting and relevant.

• In your case, if your main reason for coming together is astronomy, then that might be a way to focus. The history of astronomy/physics/mathematics (they all overlap in interesting ways) is fascinating, and goes back thousands of years. It carries with it many interesting insights into how people understand the universe and their place in it. I just bring up this thematic way of paring things down because maybe you want to focus on some fields and not others.

• If you are starting from relatively "nothing," a good thing to find is a textbook that is well-written and can give you some structure. I use McClellan and Dorn's Science and Technology in World History when I teach at the college level. It is pretty readable compared to a lot of work in this field, and a nice mix of "facts" and "interpretation," and the study of history is usually both of these together. It is worth checking out.

• I have a PhD in the History of Science and even I don't know everything about all of it. It's a huge subject spanning thousands of years. Which is just a way to say: you don't have to approach this like you need to know everything. You share what you've learned with others, and if it's interesting to you, it's probably interesting to them. Don't feel any pressure to be comprehensive. Feel free to say, "I don't know."

• The trickiest thing you'll have in this is that much of the academic literature on the history of science is not at all geared towards the age group you're targeting. It's full of fancy jargon and preoccupations whose importance is not clear. If you wanted to dip your toe into the kinds of "big questions" academics are interested in, though, Thomas Kuhn's The Structure of Scientific Revolutions is a largely very readable book, and is a very famous book that presents a "big picture" theory of how science works. It's the sort of thing that one could imagine reading over the course of several weeks for such a course.

Hope that helps! You've got a daunting task ahead of you but it can be a fun one. Here's the basic outline of topics I did in my course the last time I taught it; the "Mc&D" chapters come from the previously mentioned book. Note that these are two 90 minute classes per week, and my main subject knowledge is modern science, so I go a little heavily on that towards the end (and go beyond the textbook quite a bit, because modern stuff is not its strong point):

WEEK 1 W 1/18: Course introduction
Reading: None! Other than the syllabus! Are you reading it? You should be!

WEEK 2
M 1/23: Theories of history
Reading: Mc&D, “Introduction”

W 1/25: Pre-historic technologies
Reading: Mc&D, Chapters 1 and 2 (“Humankind emerges” and “Reign of the Farmer”)

WEEK 3
M 1/30: Babylonian science and technology
Reading: Mc&D, Chapter 3 (“Pharaohs and Engineers”)
W 2/1: Egyptian technology & Intro to Hellenic science
Reading: Mc&D, Finish chapter 3, start chapter 4 (“Greeks Bearing Gifts”)

WEEK 4
M 2/6: Hellenic science: Philosophy and Forms
Reading: Chapter ( “Greeks Bearing Gifts)
W 2/8: Hellenistic science: Alexandria, the Romans, the Byzantines
Reading: Mc&D, Chapter 5 (“Alexandria and After”)!<

WEEK 5
M 2/13: Arabic and Islamic Science
Reading: Mc&D, Chapter 6 (“The Enduring East”)
W 2/15: China and India
Reading: Mc&D, Chapters 7 and 8 (“The Middle Kingdom” and “Indus, Ganges, and Beyond”)

WEEK 6
Tu 2/21: The Americas
Reading: Mc&D, Chapter 9 (“The New World”)
W 2/22: Medieval Europe
Reading: Mc&D, Chapter 10 (“Plows, Stirrups, Guns, and Plagues”)

WEEK 7
M 2/27: Copernicus: Mathematics, religion, and cosmology
Reading: Mc&D, Chapter 11 (“Copernicus Incites a Revolution”)
W 3/1: Galileo: Astronomy, politics, and the Church
Reading: Mc&D, Chapter 12 (“The Crime and Punishment of Galileo Galilei”)

WEEK 8
M 3/6: In-class midterm review
W 3/8: In-class midterm!

WEEK 9
M 3/20: Early modern science
Reading: Mc&D, Chapter 13 (“God said, ‘Let Newton be!’”)
W 3/22: The Royal Society of London
Reading: Finish/catch up with Ch. 13, and read excerpt of Royal Society meeting

WEEK 10
M 3/27: Science and the Enlightenment
Reading: Choose any three articles from the Encyclopédie of Diderot and d’Alembert — be prepared to talk about them in class on Monday!
W 3/29: Chemical and Industrial Revolutions
Reading: Mc&D, Chapter 14 (“Textiles, Timber, Coal, and Steam”)

WEEK 11
M 4/3: From Newton to the Aether
Reading: Mc&D, Chapter 15 (“Legacies of Revolution: From Newton to Einstein”)
M 4/5: Darwin and Freud
Reading: Mc&D, Chapter 16 (“Life Itself”)

WEEK 12
M 4/10: Electrification: Theory and Practice
Reading: Mc&D, Chapter 17 (“Toolmakers Take Command”)
W 4/12: Science and technology at the fin de siècle
Reading: Mc&D, Chapter 18 (“The New Aristotelians”)
Optional: Einstein, “Geometry and Experience” [on Canvas]

WEEK 13
M 4/17: The Changing Technology of War, 1865-1917
Reading: Wellerstein, “Conant’s War”
(And catch up on any reading you are behind on.)
W 4/19: Sciences of the State
Reading: Wellerstein, “States of Eugenics”

WEEK 14
M 4/24: The Second World War
Reading: Mc&D, Chapter 19 (“The Bomb, the Internet, and the Genome”)
W 4/26: The Atomic Bomb
Reading: Wellerstein, “The First Light of Trinity,” “Nagasaki: The Last Bomb,” and “The Race that Wasn’t” (on Canvas)

WEEK 15
M 5/1: The Cold War
Reading: Mc&D, Chapter 19 (“The Bomb, the Internet, and the Genome”) (finish/catch up)
W 5/3: Science and technology at the Cold War’s end
Reading: Mc&D, Chapter 20 (“Under Today’s Pharaohs”)

To be frank with you, without a sense of the discipline, I have no idea how you are going to do this. Knowing about science doesn't necessarily lend itself to understanding the history of science. The fact that you think there are "not that many starting points on this subject" demonstrates that you really don't know the discipline.

I would suggest to you that for your first class you ask the students what they think a history of science would look like. Is it a history of famous scientists and their inventions?; or, is it a study of the historical contexts -- such as empire -- that produced scientific developments? or, is it a history of the growth of 'rationalism' that enabled empirical study?; or, is it the history of the institutions that enabled its development as a set of disciplines and a profession?; or, is it a study of the materiality of science -- the artisans who were critical in the making of tools?

As you can see, the field has been approached in many different ways. The materiality of science has been the most recent development in the historiography. You might consider approaching this class through the question of 'what is the history of science?' That would let you demonstrate how different approaches to this question have changed over time, and you can spend a bit of time on some of the most important works in each category.