Need New style

Hi everyone, I'm in need of some advice. I am 27 years old and still dress like I did in high school. My normal outfits are t shirts, jeans, and sneakers like rocket dog or converse. My bf has been commenting that I need to 'dress my age' and that I'd feel better if I looked better. How do I go about looking more sophisticated and grown up without showing off a lot of skin? I'm not overweight or anything but I have some body image issues and don't like to show off my chest or arms. Help! I really need to make a change so I'll be taken more seriously. I am on a tight budget so that's a factor as well. 😑