http://putthison.com/post/712103418/the-essential-mans-wardrobe-perhaps-the-most
a pretty good safe catchall list to work from; I'd start with the more summery shit since well, it's summer.
as far as what to buy; i'd stick with a palette of 4 or so colors that all gel together and keep it simple; avoid crazy stripes/patterns outside of maybe a shirt or sweater.
as far as where to buy; h&m, express, j crew, etc. banana republic is ok, some of the tailored stuff is not bad, but the casual clothing there is just ... not great imo. I like target for tees, not a fan of their button ups unless it's a designer collab, which fit better and are more interesting in general. walmart has good fitting levi's, the signature line, for about $20.
as far as an outfit suggestion:
business casual: harrington-type jacket, button up shirt, simple chinos, suede bucks or other light oxford shoe. go up a formal tier with a cotton or linen blazer, go down a tier with simple flat-soled sneakers (vans, etc)
you can make this even more casual but good with just some jeans, a tee, and sneakers.
poolside: chino shorts, short sleeve shirt, vans/espadrilles
don't know if that helps but i tried ;(