Menswear
Collar
Button the collar. You should be able to slide a finger or two between the collar and your neck. Tighter feels choked, looser gaps when you wear a tie.
Fit
Shoulder seams sit on your shoulders, cuffs end at the wrist bone, and the body skims you without billowing. If you tuck it in, check that it stays put when you raise your arms.
Fabric
Oxford has a slightly textured weave and suits casual and smart-casual looks. Poplin is smooth and crisp, and a common choice with a suit and tie.
Colour
White and light blue are the easiest places to start. They work with most suits, ties and jackets.
Try a few collars on in store to see which one feels best on you.