Prepare your speech
No, you shouldn’t bring your whole speech written down on paper. You also shouldn’t just learn it by heart and then mechanically recite it. But you have to know what you want to say and not forget it when feeling a little stressed out. Most of us cannot stay absolutely calm in a situation like this. Be frank and tell your boss your reasons why you want to leave. You can also thank him or her for the experience you gained when working for them.
Tell your boss in advance
Maybe you tend to procrastinate on telling your boss you want to quit. Not a good move. Why? Because it is quite disrespectful and inconsiderate if you just march into the office and surprise them one day before you need to leave. And also, for some of us, there is a notice period which has to be observed in order to be in compliance with your contract - job posting. So, definitely don’t put this (maybe) unpleasant conversation off.
Don’t overthink it
You want to be prepared for any kind of reaction, but you never will be. If you don’t know your boss very well, you will just never do this the best way possible. You might be very nervous about it and then it can help to tell yourself that when it’s over, you are going to feel very relieved and you can even reward yourself with something small. That and the fact your new work life awaits should be motivating enough.