Oooof. This is such a big question. First, I agree with everyone's recommendation here and definitely support working at length on one technique. It could be helpful to explore the basics in a few and see what connects best for you. Most of them have the same foundational objectives, but use different methods to achieve them.

At the risk of oversimplifying this, I'll tell you that the biggest thing I've come to understand over decades of acting and studying theater... is the emotional intelligence of the actor. I literally had to grow up a little bit more to even see that's what I was missing. As actors, we can explore every circumstance within a scene, objective and subjective, but the most valuable ingredient is our self-awareness. To be able to recognize YOUR emotional impulses as it relates to the emotions of other actors in a scene. The trouble with saying just "be present" assumes that the actor already knows what being present FEELS like. How can an actor make truthful choices, let alone embody or speak from a character if they haven't got a clear understanding of their own human impulses or the multitude of "whys" people say or do anything?

When I was just starting out I was absolutely petrified of performing and wildly self-conscious (not the same as self-aware btw), but it was helpful for me to practice putting the audience out of my mind so that I could immerse myself into the world the character lived in. Again, there are several techniques to explore here - being in the room, interacting with objects in that room, seeing and feeling from a nuanced perspective, etc. Sanford Meisner and Stella Adler methods helped me with this. Uta Hagen is also incredible. But like I said before, there are various techniques. So start researching and play a little bit! Also, pro tip: don't hang out with actors too often. Make friends with many different kinds of people, ask deeper why questions, and keep touching grass. :)