ANDY'S ANSWER: Wearing the correct-size bra will certainly make it more comfortable for you and make your clothes fit better as well. Your band should be snug, but not so tight that it cuts into your back. Your breasts should also not puff out over your bra under the arms or over the tops of the front of your bra. To find out what is your correct bra size, you will need a soft measuring tape.
Look in your Mother's sewing kit, not in your Dad's garage. It might be helpful to have someone else help you measure yourself, but you can also do it yourself.
Stand up straight but do not expand your chest. Put the tape measure all around your body directly under your breasts and note the number. Add 5 inches to that, rounding up if you are over a half inch on your measurement. That is your band size.
The next measurement should be taken with a bra on if you already have one (if you don't, just take the measure without one on). Measure all the way around your body, across your back and over the fullest part of your breasts (nipples). Now make note of that number and subtract your band size. Depending on what the difference is, this will be your cup size. If the number of your cup size equals your band size you will be an AA cup. If your cup size is 1 inch over your band size, you will be an A cup, 2 inches over - B cup, 3 inches over - C cup, 4 inches over, D cup, 5 inches over DD cup. There are larger sizes than that, but I'm going to assume you aren't there yet.
So... for example, if your first measurement under your breasts is 27", you add 5 to that and you will get 32. That will be your band size. Then if the measurement around your breasts at the nipple level is 33 inches, your bra size will be 32A. This is only a guideline and is a place to start. If the cup on your bra can be pushed in and stays, your cup size is too big and you should go down a cup size. If you are spilling over the side, go up a cup size.