1. The most important thing while you're on stage is to calm down. If you're worried about what the audience is thinking, you're just going to get nervous, and unless you are trying to portray nervousness, that's not going to be a good look for you.
2. In order to be portrayed as a good actor, you must respond to your surroundings on stage. When your lines end, you do not leave the scene, and you must continue to react to the scene as your character would, otherwise the whole performance will fall flat, as group acting is a team effort.
3. Who is your character? You must think about what ticks the character has, how the character sounds, how the character would act outside the play. You must step into the skin of your character and forget who you are, portraying, for the time you are acting, only this character whoever they may be. It's one of the most fun parts of acting! And this also makes it much easier to respond to situations on stage as the character if you so happen to forget your lines or you're doing improv.
4. Though it's not the most important thing if you can easily execute the above things, you must know your lines, because that is extremely important to executing a scripted play, however, if you are too worried about memorising your lines and not the above other things, the audience will likely not feel the performance and will see that you are only simply reciting your lines.
Hopefully these tips have assisted you!