My advice is keep it simple and tidy.
Op, with fine hair I have found the big clips (a bit like the ones hairdressers use for sectioning, but nicer, you can buy them at the hair clips stands in supermarkets etc) are great for keeping fine hair tidy. A lot of fine hair is too fine for a ponytail.
Simple makeup routine. I always have a dab of lipstick and mascara on as a basic.
A good neat fitting coat. Not something that looks puffy or crumpled.
Neat shoes. I don't wear trainers, what with the clompy look and the laces they are not "neat".
A neat bag! Again nothing baggy or saggy or with loads of pockets and zips.
Often I just wear one dress and a pair of tights. Or a jumper and skirt and tights. I don't wear loads of layers, or tops that cling to my tummy and boobs (even fitted t shirts).
Dancing Tatty - you asked for online shops recommendations - I can thoroughly recommend ASOS which has a lot of high street brands plus many international brands and there are usually lots of bargains and sales. Their own-brand ASOS clothes are well made and priced reasonably for the quality, I feel. For delivery, if you pay £9.99 as a one-off you get free next day delivery and free returns for a year. The delivery can be to your house or a Collect + centre which is usually a convenience store or a petrol station. I only buy clothes online and ASOS can't be beaten for their delivery options, in my opinion.
flirtygirl agree that larger people can look very stylish and well groomed. The only difference is that cheaper clothes can sometimes hang better on thinner people, and larger people can benefit from better quality fabrics and cuts. I say this as a former overweight person myself (now a size 8 after finally getting a permanent grip on my overeating habit). But size is definitely no obstacle to style as such. I read some fashion blogs and one of the best that I have had most fashion tips from is a girl who is a size 22-24. Her colour combinations and style is excellent and I must have taken inspiration from around 10 of her posts now.