As soon as you arrive on Batuu, you will notice some changes in the details of your Sim. In the career tab already, you will see the different missions in which you participate. These missions will make you gain reputation in one of the factions (Resistance, First Order, Scoundrels).
Missions can be carried out by talking to Lieutenant Agnon (First Order), Hondo Ohnaka (Hooligan) or Vi Moradi (Resistance). They are quite easy to spot since they have their group's logo above their head.
Missions usually consist of interactions with other Sims or with the environment.
Your first mission is a global mission. It takes you to discover your base camp. Once completed, you can meet with one of the factions to launch a mission and support a cause.