definately go to the shops, but a few things to know, you can quickly guess-timate your boot size by adding 18, so an 8.5 shoe would be a 26.5, and a 10 would be 28, and so on. The biggest thing to remember when getting boots fit is that your feet and legs shrink in the winter (as a result of the cold) so try fitting the boots without socks, and you should not be maxing out the buckles.
Also, another good thing to try is take the liner out of the boot, put your foot in there, and slide it forward. You should have like 2 fingers width of space between your heel and the back of the boot.
And footbeds can be your best friend. I definately believe in them, and they've helped me get a much better feel for whats going on in the boot and made me a little less dependant on the canting abilities of the boot.
As far as what the difference between boots is, mainly its little technological differences, shell volumes, and flex. Any good shop should be able to tell you about the little tech-ie advances, but as far as flex goes, the stiffer the boot, the more directly your movements translate through the ski to the snow, the stiffer they are the less comfortable you can expect them to be, however the tradeoff is better control and more accurate feedback from your feet.
Hopefully this helps, but like everyone said, the shops are gonna be your best bet, so try on everything that you can, and don't be pushed into something that doesn't feel right.