He should just go to work automatically when it's time, all Sims do. If he doesn't clicking on the little envelope icon in the interface should open up the career panel and you should be able to send him to work by clicking the button in the top left hand corner there. (Looks like a Sim sitting behind a desk).
If you are struggling learning how to play the game, Deligracy made a couple of tutorials for people just getting started, I'm sure if you do a search for Sims 4 tutorials on Youtube, they should turn up.
But honestly, just explore the userface and click on everything in the game, best way to learn, lol.