I am sure Macy's still has certified fitters on staff.
The fitting instructions are semi-complicated and to get a true fit you really need someone to help.
First, measure around your chest, under your breasts. If you get an even number like 28 add 6 to that number so you have 34, if you get an odd number like 29, add 5 to that number-same 34. That is your band size. Then measure around the fullest part of your bust. If you get the same number you got for your band size you would be a AA (34AA in this example), if you get 1 inch larger you would be an A, 2 inches B, 3 inches C, etc. Now once you get into the full figure line and some of the larger band sizes often you will measure out at your band size, say you measure out at 40", that might be your band size.
You also want the fabric of your bra to cover all of your breast tissue, the space between the cups should lie flat against your chest--this is where it gets tricky because some people don't have a lot of natural separation so they need to find a bra that doesn't have a lot of space between the cups, etc.
You should not have any tissue peeking out over the top of your bra or squirting out the sides of your bra.
For support, the more clasps you have, the more support you will get. A non-padded bra will give you MORE support then a padded bra because it conforms to your breasts better. A padded bra will make you giggle more.
The back of your bra should be under your shoulder blades even with the front of your bra. It should not be below or above the front of your bra--think straight line all the way around.
Putting your bra on the right way makes a difference too. You should actually lean over and "apply" the bra vs putting the bra on top of your breasts. You can also put on the bra and then lift your breasts into place. The underwire/band should be UNDER your breasts, you shouldn't have any peekage going on under your bra.
Now, where a fitter comes in handy, just because you measure out at a 34C or what ever doesn't mean that size in all bras will fit you properly. Some companies tend to run small/big so you may need a different size in that brand. Also the construction/shape of the bra makes a HUGE difference in fit. Some bras are more circular others are more triangular, etc. Finding the right shape of bra is a big challenge.
Hope this helps.