Try wearing your hair down: curl it, have half-up etc.
Try some make-up if you're comfortable with it on
I totally know what you mean about being comfortable in t-shirts and in jeans. I don't wear dresses because I feel open and exposed, plus I suck at sitting in them properly. I mean I'll wear one if I go out but a daily little sun dress is a no.
I would start out with just dressing up the jeans you have so you don't feel uncomfortable you know? :) Like a cute top instead of plain shirts. Like a top that hangs off the shoulders are so cute. It feels like you're wearing a baggy shirt plus it's cute and comfortable. :)