Senate electorates are multi-member, meaning there is a group of senators elected to look after the interests of all the people of a state or territory. This is different to the House of Representatives, where only one member is elected for each electorate which is part of a state or territory.
The other way you could become a senator is to be appointed by your state or territory parliament to fill a casual vacancy, which occurs if an existing senator resigns, retires or dies in office. You would then need to contest an election at the end of your appointment to continue as a senator.
Some of the other requirements to be a senator include:
The Australian Electoral Commission is responsible for running Senate elections and their website has more information about the requirements for senators, including the nomination process, campaigning and how the votes are counted on election day.