Cheaper clothes can look quite good when new but after a few washes they lose their shape / colour. Designer labels tend to source the best quality fabrics that last a long time and look better as they wear in.

My advice would be it's better to buy one designer quality item than 3 cheap ones. It also means that if you shell out on alterations to get the fit right it's money better spent. The fit is important. But it depends on your budget. Another way to save money is try to get one piece that goes with 2 other things you have. This is something girls are very good at...matching. Some store sales assistants will help you with this, especially a woman or trendy guy.

You can get ideas for style by looking at what other people are wearing, e.g. like those shoes, like that jacket, etc. You can see what works and go looking for similar things. You don't want to be completely generic but at the beginning if you're clueless this is a good way to get started with a wardrobe. Eventually you want to develop your own individual style and know what looks / feels good on you.

This is really for your own confidence when you need all the help you can get in feeling more confident about yourself. But you don't want to get too hung up on it. Just do what you can. If they see you've made an effort to look good that's more what girls will pick up on than the actual clothes themselves.