I had the same problem. I went to Tilly's and Aeropostale I got a nice shirts that I could wear any time for anything. I still got something comfortable though so I would want to dress nice.

Go to www.tyra.com she has everything! and she has real makeovers from her show with ladies with the same problem

I wore t's all the way up into my 30's. then, life changed for me 6 months ago, and then my boss said "lose the t shirts." they wanted me to wear these things they named-BLOUSES.
being sorta a tom boy most of my life, I rejected all that silly prissy stuff, preferring to wipe my greasy hands after working on a car on my nice tshirt over an expensive "blouse". (I have an automotive degree, so as you can see, I took my tomboism to the extreme) So, here I was, working in an office, and wearing a blouse. I had bought a few airy blouses at a secondhand shop ( I mean, why pay retail? I have better things to spend my money on, like a t shirt I avctually like over a blouse, right?) And, it wasn't too bad. but I realized why I preferred tshirts over blouses. they make them too short in my size, but with a tshirt, they kinda figured out that the body may have a longer torso. -+

Umm. Depending on what your style is, you could try hoodies, cardigans, vests, belts, tank tops, scarves, leggings, tights, shorts, sweats, skirts, cargo pants... the list is endless. Also there's different kinds of t shirts you know. V-neck, scoop neck, boat neck, crew, asymmetrical, off the shoulders, ones with funky cut outs. Women's fashion has SO much variety. Play around with it. Also, there are different kinds of shoes.