Reading the cereal box while eating breakfast, and reading a report for work -- or even a novel for a book club -- are very different activities. For the latter, treat reading itself as the goal. If you're interested only in getting to the end, you won't pay attention; you will understand little and remember even less. Contemplate questions about the text before you begin -- what do you want to learn from this book, for example -- and read as if you are seeking answers; this will help stimulate your brain to reinforce found answers. You're more likely to understand the text if you read something carefully once than if you read it two or three times hoping for clarity.
It doesn't matter what you read; be comfortable reading various materials, from newspapers to textbooks. Good readers are efficient, knowing when to speed up for easier material and when to slow down when things get complex. The more you read, the more your brain understands about source material, language usage, structure and organization. With consistent practice, your brain will learn to anticipate what types of information are coming, which helps with comprehension when you get there. Try reading for a minimum of 20 to 30 minutes daily.
The more you read, the more words you will recognize, filling in their meaning in context. The more words you know, the better you can comprehend what you are reading simply because you know what the author is saying. You also will read better because your brain will not slow down to grasp for the meaning of a vaguely familiar word or stop short to look up a word. Starting and stopping break concentration and result in your reading less than you would in the same amount of time if you had not paused to open the dictionary.
Beginning readers look at one letter at a time to figure out the word. Poor readers look at one word at a time and in doing so lose time and comprehension. Read text pieces as a unit instead of mentally pausing -- however briefly -- after each word. Work up till you read to the next punctuation mark, for example. Often, this will give you a phrase or concept better comprehended than individual words, which would have to be added together to be understood. Over time, it will become easier to identify which phrases, sentences or paragraphs merit scanning, skimming, casual reading or studying.