There is no one answer that will suit all people as we all have varying skills and learning abilities. As to whether 'it is easy' will depend on some degree to your intellect or learning ability as some people grasp the principles straight away but others can be told the same things a 1000 times but still not grasp the concepts (mentioning no names).
There are many courses around but the good ones do come at a price and there are a number of css courses on Sitepoint for html and css. I would start with a structured introductory course on html and CSS and then move on to more advanced topics once you understand the logic and terminology.
CSS these days is a vast topic and its easy to get overwhelmed so its best to start with the basics and then gradually increase your knowledge over a period of time. Once you have a grasp on the basics then the rest is easy. It's just practice,practice and more practice.
Write your own code and even if you copy and paste be sure that you understand what you are copying and pasting. It does not aid learning to simply copy and paste but writing your own code always has great benefits.
Use the w3c css and html validators every time you write a block of code to make sure it is correct. There is no point in learning how to write broken code.
Buy a few good books to keep by your desk or for reading when offline as they are useful when you need to lookup or remember something. Do a search of this forum for best books as the question has been asked a number of times.
I learned most of my stuff (about 2 decades ago) by browsing web developer forums like Sitepoint and reading all the posts that interested me and trying to understand and then improve on the answers. These days every question you need to ask has probably been solved a 1000 times already so you just need to google it.
Once you have grasped the basics on the disciplines you want to learn then it is probably time to specialise in one area so that you become very good in a specific area (without forgetting all the other stuff) and gradually build a good reputation for that specific discipline.