Interesting question Liliana. Although most bills are introduced in the House of Representatives and then sent to the Senate, bills may also be introduced in the Senate (except for money and taxation bills) and then sent to the House of Representatives. A bill is passed by either chamber by a majority vote. This means a majority (half plus one) of members or senators present vote to pass the bill in either a vote on the voices or division. Not all members or senators need to vote for the bill to be passed.
Want more information about the law-making process? Check out this fact sheet about how laws are made in the Australian Parliament.