I suffer from anxiety and depression, am pretty certain I'm autistic and I hardly ever go out anywhere (just to explain why I'm in a panic about something so trivial).
I've been invited out in less than a fortnight. This will literally be my third night out since DS1 was born four years ago. It's the birthday of DH's boss's girlfriend (though, she also works for the same company so DH is friends with her as well, if that makes sense?). I believe she's turning 25. She's lovely and I've met her a few times now but she's also very confident and I guess I find her a little intimidating because, despite the fact she's younger than me, she seems so much more 'grown up' than I feel.
The venue is - I think - a club in London. My teen years/introduction to nights out were spent in rural Scotland so that's also intimidating! 
I have no idea what to wear.
My 'style' is...sort of hippy (If I can be said to have a style) but I'm quite eclectic with clothing I think.
I'm a size 8 on top. Maybe even a 6 sometimes though I have a bit of a 'tummy' these days but I'm very bottom heavy so am probably more like a 12 on my bottom half.
I'm not graceful at all so big heels are a no.
I have dark brown eyes and dark hair, which is really not styled right now. Basically, I decided on a clean slate hair-wise a few weeks ago and got DH to shave my head (not bald - to a 'number nine' or something).
I ordered a dress that looked lovely in the photo despite being dirt cheap but it arrived this afternoon and looks every bit as cheap as it was.
I'm normally quite excited about getting dressed up but I'm just lacking in confidence these days.
Any ideas on where I can start? Style of dress/outfit? General advice? We don't have masses of spare money but DH did do a LOT of overtime last month so we will have some spare money coming in and DH has just ordered himself an expensive piece of computer equipment so I think he'd encourage me to buy something new if I wanted. Online only, though, because I have two young autistic children so shopping is not an option.