I want to improve my learning skills. Is there any advice or a good method for that? For example, I want to read an article or book and retain as much information possible. But sometimes I read and lose focus, or when I read I can't remember everything as well as I'd like to.
Long answer Practice (smart)! The more time you spend being a student (studying, reading, summarizing, asking questions, taking notes, and pursuing answers) the better you will become at being a student. For more specific tips, check out this article by Scott Young. He focuses on methods that allow you to study smarter rather than harder. The key to this is to recognize that all knowledge is interrelated; everything connects to everything else, and therefore no learning is wasted. I find his tips and hints to be useful, though I cannot learn quite as efficiently as Young claims one should be able to.
Something to look into is style of learning. For example, I'm a haptic learner, which means I learn best through doing. 'Doing' in this case can be a simple as making flashcards. On the other hand, I can try and read a text book all day, and I won't retain much at all. Therefore, 30 minutes of making and drilling flashcards is a much more efficient use of my time instead of an hour spent reading.
Everyone learns slightly differently. If you learn best when you hear something, try recording the lectures and playing them back. If you are a visual learner, maybe look into plotting out lecture points on a white board. Honestly this is just something that you need play around with, though you'll find many ideas to get you started on the internet.