I've bought a lot of T shirts from Everlane, ~$18-28/shirt. They have a few different fabric weights. I suggest you grab a couple to see which you prefer. They change colors often which I've liked. To start, stick with grey, black, and white. You cannot go wrong with those colors.

I'm 6'1" 165 lbs and medium fits me well. Get a tailors tape measure. You can always email/call higher end shops for help. Be prepared to return things. The fit will make or break the look.

2 pairs of well fitting jeans (lets say one medium blue and one black), those t shirts, a couple decent SIMPLE sneakers (1 black 1 white), and a pair of simple dark brown leather boots and you'll be SET. Get a simple black bomber jacket before fall hits.

Just to throw out some other brands and stores I love (although the prices are on the high end) 3sixteen, Withered Fig, Dehen1920, Division Road, Shockoe Atelier, Rag and Bone, Railcar Fine Goods, Gitman Vintage (amazing button down shirts, USA made), Blue Owl Workshop...