How do you know what looks flattering on you and what doesn’t when you think only jeans and some sort of T-shirt look good on you?

I’m wanting to buy clothes that are out more of my comfort zone and really want some different style of clothing to possibly help with my overall body/self image confidence, but really have no idea what will possibly look good on me and what won’t. Has anyone else ever gone through this? And if you did, how did you figure it out?