Copywriting is some serious stuff. "Copy" is basically the industry term for words, so a copywriter can be asked to write potentially anything for a company. Often, they write "print materials" like brochures, flyers, pamphlets, etc. They may also be writing the web-based materials, too.
A good copywriter should also have a strong sense of design. As such, they should know at least the basics of actually making those materials, including the software used to lay everything out (no, Microsoft Word is not the answer).
There are countless design books out there. I got my first taste of it from The Non-Designer's Design Book by Robin Williams (no, not that one). Once you get a feel for the principles and the basics of layout and design, you can derive a lot of what is and isn't acceptable.
But the point of this post is to cover an intersection of copywriting and layout/design that is often missed by both sides: intent.
When you make any document, the absolute first question you must always ask is: What is this document supposed to do?
The words you choose to put down must serve a purpose. If you don't know what that purpose is, you cannot write a successful document.
And the answer is very unlikely to be "informing people". Written materials are supposed to make people do things. Consider the classic 3-panel brochure. Let's say it is one for a tour company based in Niagara Falls. What is the brochure supposed to do? It is supposed to entice tourists to go take a tour of the falls after lunch. Because of that, the brochure needs to have certain information: times, prices, starting location. It will need a good pitch, flashy descriptive writing, and some attractive pictures, too, but if they don't know the basic information, then your brochure is malfunctioning.
Now consider a month-long summer camp. There will be way more information about the camp, its activities and offerings, and its various pricing options. The only way to really tell them everything they need to know is by having interested parents check out the camp's website. Therefore, the brochure is supposed to intrigue people and compel them to look online to satisfy their curiosity. This brochure also needs good pictures and flashy text, but it now it needs to tell you a little bit about all of the different aspects of the camp (eligibility, activities, history), always suggesting they check out the website to get the full story. This brochure is not supposed to get kids to register right away; it is supposed to make them do more research.
It seems really simple and obvious, but that's because I just told it to you. I constantly find this overlooked (yes, I am a big enough nerd that if I'm waiting around somewhere, I will read the brochures and tear them apart both as a writer and a designer). If you want to get into copywriting or design and layout, never forget that question; it will solve far more problems than any other piece of advice you will come across.