Confused about referencing for an essay

Apparently you have to put in reference for an idea you found somewhere. But literally everything I write would have come from somewhere where I've read up, i.e a book or a website, in other words they're not going to be something I came up with by myself.

I don't understand how referencing works, because going by that, wouldn't I have to put in '(author, name of book, year)' at the end of every single sentence in my essay. That doesn't seem right, and in every essay/academic writings I've read through my years in school and uni now, they include references every now and then, but not at the end of every sentence.

You need to put in references for every fact/ quote/ particular person's opinion etc. however you should interpret them yourself and perhaps discuss your own thoughts on the topic (obviously depending a bit on what subject you do though...).

That depends on your referencing system, e.g. the Harvard referencing system is 'Name, Year' in brackets in your essay followed by a much longer load of information in your bibliography. Other systems just use numbers in your essay (Wikipedia style) and again include all the information in the bibliography. If your teacher hasn't insisted upon a particular method, I would advise doing a bit of research yourself on the different options and picking the one you personally prefer. It again depends a bit on your subject/ essay topic too - for example in an essay for subjects like Philosophy where the person who came up with a particular idea is quite important something like the Harvard referencing system would be useful, whereas for a scientific essay using facts from a wide variety of sources where it doesn't really matter which scientist discovered them the numeric referencing system would clutter up your essay less and make it easier to read.
If you are typing your essay using a fairly recent version of MS Word, I would strongly advise using their built in referencing system - it is surprisingly good, automatically creates bibliographies and rearranges sources appropriately, and also allows you to quickly change your referencing system just to see what each would look like to get more of an idea of which would be most suitable (and definitely reducing the effort if you change your mind part way through)

What do you mean by idea? Other people's ideas you should probably reference

my essay is pretty much scientific, its about comparing the properties of different materials, so I would imagine its all going to be facts, no room for my own interpretation. Does this mean that there would have to be a reference (a small number: wikipedia style, or: author, year - havard system) at the end of virtualy every single in my essay?

There is probably still a bit of room for your own interpretation - e.g. if you are focusing on a particular application you explain why a particular property of the material makes it suitable.
I personally would definitely go for the numeric (Wikipedia style) system for this kind of essay, but be careful only to reference the actual facts (i.e. don't stick numbers in every sentence, only when you have used a particular bit of information from that source - so if you then go on to discuss that particular property in more detail or use a quoted formula in an example you would put the number straight after the fact):
e.g. '... has a high tensile strength [1] which makes it suitable for the application of ... since the material must resist bending in order to provide structural support'
Here, I clearly show which fact has been taken from the source, followed by my own interpretation of how that fact can be applied to the particular example I am discussing.

thanks for providing an example

but what if theres a book(s) which also makes the same interpretation as the one you made, because for something like in your example, theres not much room for 'original' thought/interpretation, chances are what your interpreting would've already been written down and published in some other book. So isn't there still room for being accused of plagiarism?