The first thing you need to do when looking to expand your shirt empire is check your measurements. Grab a tape measure and quickly record a few key stats about your body shape to make sure you're on the mark every time. The most obvious measurement is the collar. You need to get this right, as it can throw off the entire look if you get it wrong.
Shirt collars are measured in inches and usually run in half sizes so you can get the perfect fit. If you have a 15-inch neck measurement you should choose a 15 and a half inch shirt collar to allow yourself a bit of extra breathing space. All you need to do is wrap the tape measure around your neck (not too tightly but with no gaping) and read off the size, then add half an inch to that. If you're between sizes, you should round up for comfort.
Following on from the collar measurement, you can check your chest, although this will vary from shirt to shirt. Formal styles tend to be sized with the collar first, whereas more casual shirts like Oxford cotton styles usually have regular S, M, L sizing, so its best to try a few on a see which feels best.