While in a dojo, the way you sit is important, especially if the Sensei is strict. The way and Aikidoka (aikido student) sits is known as "seiza". To sit in Seiza you get down on both knees and then sit yourself down.

There are two ways of doing this, you could sit on your heels as shown in the following pictures.

You could also flaten your feet, making the first toes of each foot touch each other.