# This generation is an evil generation. It seeks ... to it

Here "generation" refers to the people in it. Alternate translation: "The people living at this time are evil people. They seek ... to them" or "You people living at this time are evil people. You seek ... to you"

# It seeks a sign

"It wants me to do a miracle to prove that I have come from God"

# no sign will be given to it except the sign of Jonah

"The only sign that will be given to this generation is the sign of Jonah"

# no sign will be given to it

 "God will not give it a sign" 

# the sign of Jonah

"what happened to Jonah" or "the miracle that God did for Jonah"