Hi Jemster, I know exactly how you feel. I could have written your post last year, having had two children fairly quickly. I would wander round shops having no idea what was in fashion, and come home empty handed. Plus, I just looked washed out and worn out, wearing clothes I hadn't updated in about three years. I took action, started exercising to lose some weight (this wasn't helping with my general self esteem etc), and did the following:
Make up: started watching Lisa Eldridge and Pixiwoo for tips and ideas (google both, they are really good). Also read Caroline Hirons blog for skincare ideas. I threw out all the old make up. I had, and each month bought something new. I went to Mac and Estée Lauder for advice and ideas, both were great (Mac for blush, eyeshadow and lipstick, EL for foundation). I also spent a small fortune in Boots, and have come to the conclusion I will buy less makeup, but stick to the better brands for the essentials. Mascara, lip liners etc I find L'oreral and Bourjois to be good. Another thing I did was start getting my eyebrows threaded, and it was a revelation. Really makes me look more groomed. I also give myself a manicure on a Sunday night while watching the tv!
Hair - I get this trimmed and coloured every 12 weeks or so. I use an intensive conditioner once a week, and I bought a wand to given it a loose curl, and again, looked at some blogs to see how I could update my style a little (the small things blog is good for this)
Clothes - again, looked at some blogs, and particularly, Pinterest, to see what things I liked. Also, once the kids are asleep, I take a look on some shop websites for ideas. I threw out all the clothes that were falling apart, washed out or didn't fit, then started to buy a few basics each month. I try to stick to neutrals, and found this made a difference, as now most things in my wardrobe match unlike before! I bought some nice leather bags on eBay, and I find these can really dress up any outfit.
I hope this helps, it is horrible when you know you want to change things,but don't know how to do it. Sometimes you just need to make some time for yourself, if you don't treat yourself who will! Good luck 