Firstly, Mazal Tov!
When a couple decides to marry, they announce the occasion with an engagement party. In Yiddish this event is called a vort, which means a "word." At the vort the man and woman traditionally give their "word" and formally commit to marry.
There is a custom to break a ceramic plate at the vort. This symbolizes the seriousness of their commitment to each other: Just as breaking the plate is final, so too the engagement is final and not easily terminated. Breaking the plate also tempers the intense joy of the occasion, similar to the glass that is broken under the chupah. It reminds us that the Temple is not yet rebuilt [readers, see below].
Customarily, the couple's mothers are the ones who break the plate. They hold the plate together and drop it onto a hard surface. It's important to wrap the plate well to ensure that no one gets hurt from the broken pieces. Some have the custom to make a necklace for the bride from the broken pieces after taking the necessary safety precautions. Others give the broken pieces to eligible "singles" as if to say, "May a plate be broken for you soon." Some break the plate at the wedding just before the chuppa.