In a real sense, trying to excel at copywriting is very similar to building a successful business; it requires hard work, knowledge and never giving up on yourself. You cannot do it with a halfway effort - that will never work because it is something that does not respond to it. All pros in sports tell you it is solid execution of the fundamentals that are so important to winning; same goes for copywriting.
What you write is determined by what you want your copy to do, and it is selling something but not always a product or service.
Some words will be used differently in different contexts, and that means you seriously need to sometimes think about each word you use. You will have your clients to satisfy, and you must do that if you want to get paid or be hired again by them; so those basically are your marching orders as a copywriter.
Yes, there are differences in the reading audience whether it is for online or offline, and you will need to learn those differences. How you present your copy, or rather how you write it for any niche will depend on who is in that niche audience. Anything you ever write will have to bridge the gap between your offer and the audience, and you need to know how to do that.
So you have to keep that bit in mind if you write for both offline and online. Very many people think they can learn copywriting in a forum or on a few websites offering free lessons, huge mistake.
Never approach copywriting like you are writing a research paper in school, and even the structure and format are different. Your copy needs to have a soul, which makes crafting a worthy copy a creative process to reckon with. That is why it is so important to really know copywriting concepts, principles and devices so you can effortlessly use them. Most if not all of what you will write will get its cues from the audience demographics and the nature of the product. So look at copywriting as an effective creative process, but at the same time make sure that you're not going overboard with it. Try to avoid thinking copywriting is terribly hard because after all, it is only writing; but it is challenging. Some principles are best left to later on after you have some experience because the learning will be deeper. It is common for a lot of the better copywriters to have an attitude and air of superiority; whatever, just make sure you learn and practice.