How can i make relationships not so serious?

like i was talking to the girl i like and she likes me but we arent going out officially and the reason i told i wasnt ready for a relationship was because i dunno if i was the right guy for her but its bs obviously, then she said "u act like we are getting married" which kinda through me off cuz it kinda felt true but i have nvr had a gf. she would be my first! how can i make it not such a serious serious relationship