Number 1: Deep breath.
Number 2: Check the home page for the RULES. You'll see it stated there:
Rule Number One: Have fun with fashion! THERE ARE NO OTHER RULES.
So, let's begin. Once you've got your hair in order, and your underwear, and you know your body type, here is what I would do.
Identify a couple of your own favourite colours; ones you think look good on you or you have received compliments for wearing. If you like, snap a picture and ask the forum. Sometimes it is difficult to tell skin tone from photos taken in artificial light, but we aim to please!
Identify three or four clothing items that you'd like to start with. In your case, since you've been in jeans and sweats, how about some better fitting, better quality dark denim jeans, a couple of basic tops, and a nice cardigan or jacket?
Look at what is out there. Look at the WIW posts here, pay attention to the women's body types; you can learn a lot about what might work for you. Look at some magazines. Look in store windows without buying. Go into a store and make a deliberate point of trying on a whole bunch of different styles without the intent to buy. Leave without spending anything (if you can!) Pay attention to texture, cut, drape—what makes you feel comfortable or happy? What makes you feel awkward or out of your element?
Also, ask here: if you know your body type, ask for jeans recommendations for that body type. If you want a red cardigan, ask if anyone has seen a nice one.
You can do this. It feels overwhelming, I know. I started from a similar place. But you will amaze yourself with your new style in no time!