Sort through your current clothes into what you wear and don't wear. Then make a list of items to buy before buying anything. It's so easy to just add things to your basket and it soon adds up. If you have a list of what you're going to buy then it's easier to keep the cost down. Think about having a "core wardrobe" made up of items you'll wear often (e.g. a pair of nice fitting jeans, a couple of nice jumpers etc.) you should then be able to mix and match these to create lots of different outfits. You can then add things to this over time. If you really like something that's "fashionable" then buy it but for the most part, if you try and make sure your core wardrobe is the stuff that isn't the latest fashion but more timeless then it won't go out of fashion and you'll wear it more and for longer. It's an initial outlay but if you think about and plan it and buy items to last then you'll end up saving more in the long run and have clothes that'll last you.
In terms of where to buy, you can find some great stuff in charity shops for pennies, depop is pretty good and so is ebay. You can always look on brands websites to see what there is/what you like and then search for it on ebay. Supermarkets can also have some decent stuff at reasonable prices, I have a pair of supermarket jeans that have lasted years! H&M is also good especially for t-shirts, their basic range is really decent, and look in sales, especially for summer stuff which shops will be starting to get rid of soon.