Reading is the best way to improve many things: reading comprehension and speed, certainly, but also grammar and vocabulary. If you read modern novels with a lot of dialog, it also helps you improve your speaking. If you “read” audio books, you can even improve your listening by reading! (If you live in the U.S., it is best to borrow audio books from the public library; the ones that you can buy in a bookstore are usually abridged, or shortened, versions of the paper books, so if you are reading as you listen, you may get confused when the audio book skips something.)
You can read anything that interests you: newspapers, magazines, novels, short stories, graphic novels, children’s and young adult books, biographies, history and other nonfiction, and websites are all good! If you start something but find it too difficult or boring, stop and choose something else. Remember, this is reading for pleasure. If it isn’t pleasant, don’t do it.
Your reading material should not have so much new vocabulary that you need a dictionary to understand every sentence or paragraph or page. If you have to look up too many words, reading will not be a pleasure, and you will not want to continue doing it. Choose materials that are easy enough for you to understand the main ideas without your dictionary, but which have some words that you don’t know. You will begin to learn these new words naturally, even if you don’t look them up in the dictionary, as you encounter them in different contexts. It’s okay to use your dictionary occasionally, but it’s not necessary. You are not going to take a test on what you read, so if you don’t understand every detail, it doesn’t matter. Remember: this is reading for pleasure.