It's important to make sure that your bra fits well - an ill-fitting one isn't just uncomfortable, it can lead to back and shoulder pain, poor posture, and it tends to look bad as well.
Finding your bra size isn't hard - it just requires a measuring tape and a little simple math.
Note: There are (kind of goofy) illustrations of boobs in the following steps; if they offend you, I'm sorry in advance. If it offends you that they're just drawings (and not real boobs), sorry about that too.
Step 1: Measuring the Band Size
To determine the band size you need, measure around your chest just under your bust (just under the breasts). Make sure that the tape is against your body and straight across your back. Exhale as you take the measurement, but try not to pull the tape too tight (it should not dig into your skin at all).
Round all fractional measurements to the nearest whole number. If the number you get is even, add 4 inches; if it's odd, add 5 inches - jot that number down, it's your band size (the 34 portion of 34D, etc.)
When measuring, it may help to have someone else measure you to ensure that the tape is not twisted, and that it's parallel to the floor around the back.