It's already been said a lot in this thread about how important it is to make sure what you wear fits right no matter what it is: Preme, Bape, Stussy etc so I won't drag on about that, instead I'll try to help you out on what to actually wear. As for fits, you'll be safe if you wear a clean gingham shirt on top of a tee that cuts lower than the shirt, this is an easy wear with good layering, plus it goes with jeans or shorts. Alternatively you could keep it real with a hoodie, if you gotta wear some fresh shit then the Bape hoodies are dope, especially purple shark imo, although I don't fuck with their shorts or jeans. If you're looking for something more subtle then the Supreme bogo is a nice option because of its simplicity, yet purely because of the brand it's still really noticeable. Hoodies are more for winter though.
In terms of where to buy, you can get some super clean and dope stuff over at endclothing.co.uk, I get quite a bit of stuff there so I definitely recommend that. I also love trying to find dope pieces from fairly standard brands that go well with other clothes that you may have which stand out a bit more, I like to search for stuff like Lacoste or Polo RL as simple filler clothes. It's nice to try and find a brand with a store somewhere near you that isn't really well-known but you fuck with it, it means you always have a go-to place when you're looking for a piece to finish off the fit, for me I have an allsaints.com outlet not far away where you can get some mad bargains, I picked these up for £30 last week after a bunch of mark-downs plus an extra 30% off everything in-store, you don't get that type of shit with brands like Preme.
http://www.allsaints.com/men/footwear/allsaints-rift-crescent-low-top/?colour=352&category=315. Anyway hope I helped you out a little, I like trying lots of different fits.
Most importantly wear what you want, not what they want.