Once you reach Rank 101 you will be allowed entrance to the Senate, however, it is advisable to wait until you have at least a few hundred SPQRs to spend on maintaining your position.  Seats in the Senate incur daily costs and if your housing is not producing enough SPQRs to maintain your status, you may lose your seat.

 

To enter the Senate, click on the Senate button on the bottom right of your screen. The button sits between the Glory Ranking button and the Empire Map/City Suburb button.  From this screen, you will be given the option to enter the Antechamber.  Once you are in the Antechamber, you may close the list of names and then make your first move into the seats of the Senate.

 

Being in the Senate gives you access to extra goods you may need via the Tribute Hall and offers an additional bounty of Imperial Points per day based on your seat in the Senate.