You have to go to a station within 15ly of Sirius where Sirius Corp are present. A mission will be offered to you to go to Sirius. There is some RNG involved, so the mission may not show up straight away. After 2.3, the missions will show up more regularly.
What others said, namely you need to be in a station or base that has a Sirius Corporations faction present to get the mission. I only just got mine this week and as I was in a rush ended up switching between solo and open modes until it popped up. Don't like doing that but if you only have limited time to play it's the only way! Have patience young Padwan. ;-)