To impress your crush, you want to show that you are interested in her. Ask different things about her. The more that you know about her, the more that you can think of possible topics to talk about. You may come across some common things that you both like. You can exchange ideas for some time.
Take note that complimenting your crush the right way can definitely impress her. If she feels flattered and not offended or overwhelmed with your compliment, then this is a good thing. The more comfortable that you become with each other, the more impressed your crush is going to be.
You can always impress your crush by making sure that you are going to talk about things that are interesting to the both of you. Perhaps you both like books. You can open up a topic that will make it obvious that you know what you are talking about. It will definitely get your crush’s attention. Before you even realize it, you may exchange ideas for hours.
The topic can also vary depending on the things that you both like. You can even make comparisons about the things that you both like. Keep track of the things that you do not agree on too. Do not pretend to like something when you know that you do not like it at all.