The most useful historical reading you can do is to read The Spartans by Paul Cartledge, who is the leading authority on Sparta – it’s full of interesting anecdotes and descriptions of Spartan kings and generals in this period.
Watch films set in ancient Greece and ancient Rome, including Gladiator and 300. Although not always entirely accurate, they will get you thinking about how people lived in ancient Greece and Rome. The TV series Rome is also well worth watching.
Read some historical novels set in ancient Greece or Ancient Rome – books by Simon Scarrow, Ben Kane, Madeline Miller, Lindsey Davis, Conn Iggulden. Caroline Lawrence’s Roman Mysteries was written for teenagers but is still full of good research.