# they have kept themselves sexually pure

Possible meanings are 1) "they have not had sexual relations with a woman who was not their wife" or 2) "they are virgins."

# follow the Lamb wherever he goes

"they do whatever the Lamb does" or "they obey the Lamb" 

# redeemed from among mankind as firstfruits

"purchased out of the midst of the rest of mankind as a special celebration of salvation"