Stick to the advice we were given as young girls:
Make sure colours match or complement each other, not only for clothes, but shoes, handbag, etc.
The other piece of advice I was given, which has proved useful over the years was, buy one good skirt or pair of trousers and lots of blouses or jumpers that go with them. That way you avoid wearing exactly the same outfit all the time.
I second the advice on charity shops, but taking up dressmaking is quite a good way to look smart on a small budget. It makes it easier to find a style that suits you and has the added advantage of making it easy to tone down or disguise features you feel less than proud of, if any.
Your local library will have loads of books on basic dressmaking or can order them for you from another library.