Messages are green since I updated my iPhone to iOS 18

ever since I updated my phone to IOS.18 everytime I try to text someone it’s green. Or in other words sms but I need it to be blue in order to text someone, however when someone texts me first it fixes (magically) and it goes back to normal for a few minutes then it’s back to green/sms. Apple please fix this I wanna text my gf and I can’t 🙁‼️

(If my grammar isn’t fully correct don’t mind it please☹️)

[Re-Titled by Moderator]