The first season deals with Caesar and his overthrow of the republic. The show opens with Caesar nearing the end of his Consulship in Gaul while Pompey is having trouble with the Senate and begins to turn on his co-Consul spurred on by Cato. There is lots going on in this series and politics plays a big part in what happens, but the action is humanised by two characters who find themselves at the centre of events. These are Lucius Vorenus and Titus Pullo both soldiers in Caesar's army.
This show is not for the faint hearted it is rated R and contains lots of swearing, nudity, and violence but it does make you feel you are in Rome. There are lots of little touches that help to make the show believable, for example background actors working at various tasks are actually tradespeople. If there is a butcher in the scene then the person playing them is really a butcher. Little things but they help to make you believe that the characters know what they are doing.
I would suggest that people who have enjoyed other HBO dramas, like The Sopranos or Deadwood will enjoy this, also people who are interested in this period of history will enjoy it, just be aware they do take liberties with historical events to fit the narrative.