So I looked it up and at level 5 you can unlock your first GTA 5 mission (Lamar's mission, Ballas to the walls) and I'm level 6 and Simeon called me once just to say he will give me jobs but nothing has appeared yet. Is there something I'm doing wrong?
If nothing appears automatically yet (it should after a while), you can try to call them directly to ask for jobs via your phone. If they say that they have nothing available, just wait for an hour or so and try calling them again.
You can see the list of current jobs being offered via your phone under "job list" (in case you missed it).