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.
Print-wise, you should exercise common sense. Tropical is going to be deservedly massive this summer, but you may want to swerve it in favour of something a little plainer like a bleached wash or a light polka-dot.