hammer ons are just when u play a note, then while the note is still ringing you hit another note without struming.

for thunder struck i'm pretty sure you play an open B then hammer on the 4th fret then pull off to open again, then do the same for the 7th fret insted of the 4th fret.
i think from memory it looks like:

B |---0h4p0--0h7p0--| etc.

if someone can explain it better, please do.