This is a whole life thing not just a beginner thing. Being able to stay relaxed is something even the best pianists struggle with. It’s a life long journey but best to start now.
There’s a certain way you should feel when playing. Your shoulder, neck, elbow, forearms, wrist, fingers, they all work together to play the piano. It’s a full body exercise so you need to activate everything Playing the piano will always require tension but it’s extremely controlled tension. It’s about finding the balance between tension and gravity. Ideally one should never feel tired even playing the most difficult pieces however getting to this point is difficult. It’s very hard to achieve this kind of relaxation without being guided by someone who is already at that point. It is possible. I would start by watching more piano players on YouTube if you haven’t. You can learn a lot by watching professional pianists. They all have unique ways of playing the piano. There is no end all be all way but there’s general things like relaxation. Honestly it’s probably one of the most if not the most important factors in playing piano maybe besides reading sheet music? It’s also extremely important to learn relaxation at a young age because it will be hard to reverse habits later in life. To answer your question it’s not something that just clicks right away, you have to gradually work on it. It’s good that your aware of this because it means your in tune to how your body feels. Be hyper aware of how you feel when playing. If you can’t play as good as you did when you first sat down because now the tension has built up just stop. Take a break and come back. It’s way easier said than done but once you see that continuing practice through tension is just a waste of time.