Most Helpful Girl
I'm trying to do the same thing.
1. Style your hair. I didn't realize how big of a difference this makes to your looks until I started doing it.
2. Put effort into getting dressed. Even a comfy t-shirt and yoga pants can look stylish if you try to make it look cute and don't have the mind set of "ugh, I feel like a slob today".
3. Wear makeup, even if it's just a little mascara and blush. If you're OK with wearing more than that, watch Lauren Luke's videos on Youtube. She has a million looks for any and all occasions and explains how to do each look step-by-step.