What does 'reading critically' mean and how can I do it?

That's a really great question and a really important thing to be thinking about if you want to be an excellent English language student! If your teacher is asking you to read critically, they're probably doing so because it's one of the six subject aims and learning outcomes for the GCSE English language curriculum. According to the curriculum, reading critically involves lots of things, including "comparing and evaluating the usefulness, relevance and presentation of content", "drawing inferences and justifying these with evidence", and "reflecting critically and evaluatively on text". Each of these points are extremely important, and we can discuss them too if you like.
When thinking about how to read critically, I think it's helpful to imagine that there are roughly two ways to read something. The first way is to hold the text in front of you and look through the words without thinking too much about what the words are actually saying (like scrolling through a newsfeed). The second way is to look through the words on the page, but 'engage' with what they are saying by thinking about them a lot (like if you're reading an article on a topic that really interests you). I'd say that you can 'read critically' if you're reading the second way, but not if you're reading the first way; reading critically requires you to think about the words you're reading and what they actually mean or do. A good test for whether you're reading in the second way is to ask yourself 'do I think about the text even when I'm not reading it?'. If so, then that's a sign you can read the text critically.