Take less than a kezayit (the volume of one olive) of the karpas, dip it into salt-water or vinegar, and recite the following blessing:

Blessed are You, L-rd, our G-d, King of the universe, who creates the fruit of the earth.

When reciting this blessing, have in mind that it is also for the bitter herbs (of maror nad korech, to be eaten later on).

haggadah Section: Karpas
Source: www.chabad.org