When hearing thunder we say: Boruch attah ado-nai elo-heinu melech haolam, shekocho u’gevurato malei olam.

[Blessed are You, Lord our G-d, King of the Universe, Whose strength and might fills the world]. 

Upon seeing lightning: Boruch attah ado-nai elo-heinu melech haolam oseh maaseh bereishit.

[Blessed are You, Lord our G-d, King of the Universe, Who makes the Work of Creation].

These two blessings are recited only if you first see the lightning and then hear the thunder, or vice versa. If, however, you simultaneously see lightning and hear thunder, you just say one of the blessings, which ever one you wish, and that covers both.

One blessing suffices for all the thunder heard and/or lightning seen as long as the clouds have not cleared. If the skies clear and a new storm approaches, you have to recite the blessings anew.