Short-sleeved shirts are tricky to say the least. Get it right, and you're basically the king of everything summer, but get it wrong and you run the risk of looking like Homer Simpson.
They can be pretty unforgiving, so the trick is to get one that fits really well — you don't want it too baggy and you don't want it painfully tight. It's worth thinking about what you'll be wearing on the bottom and matching the shirt accordingly. Skinny jeans your thing? You'll need a snugger fit. But if you're going for loose jersey shorts, your shirt can be just as free and easy.