Strap fins and, more likely than not, a *smaller* boot than your street shoe size, but the only way to know for sure is to try them on.
Don't try on a pair of boots and say they fit just right, even if you think they do. Try on the same boot in the next smaller size, or the next bigger size if they really feel tight. Try on at least a couple of different models. When you take your weight off the boots (in the water), they get a little water inside them, and are being worked around inside the foot pocket of a fin, they will feel at least a little bit looser than they do standing on them in the store.
You don't want them so tight they restrict circulation (makes for cold feet and restricting circulation is not good), but you don't want them to be wobbly, either - the idea is to transfer energy from your legs to the fin, and anything that slips in between (foot to boot, boot to fin) reduces your efficiency and control. You don't want to go the way of the rock climbers of yore, who got shoes so tight they hurt all the time, to provide better "control." You don't need anything like that. But if the boot feels loose in the store, it will certainly be *too* loose in the water.
Take the time to try on as many types and sizes as you need to until you have a good fit.
