Practically nothing, just watching the tutorials on youtube.
Then the first thing I recommend you do is learn how to perform mentalism correctly, which you will not learn on YouTube (No one teaches theory since it doesn't generate views). In fact, probably just go ahead and forget everything you've learned there for now.
Pick up Bob Cassidy's Fundamentals, and Ken Weber's Maximum Entertainment 2.0. Start there.
When you learn how to properly perform mentalism (which is a very different approach than performing magic) you will realize that there are really no 'tricks', and that once you've learned a particular skill it will be with you at all times. On top of that, "powerful" comes from the performer not the method. What is powerful in my show would probably flop in someone else's, because it wouldn't fit.
If you're just out to perform tricks, then you're after mental magic, not mentalism.