I found it easiest to do this in the military career as the boss, Marcel Gattus, is single and most of the other careers' bosses are in relationships making it more difficult to become involved with them.
You can use a male OR female sim to do this it doesn't matter.
To make it easier you may want to create a new sim with the traits: Charismatic, Flirty, Hopeless Romantic and Good Kisser.
You can easily join the military career by going to the town map and selecting it on the City Hall.
After you have your new job, go home and try to invite Marcell over using the cell phone. If this doesn't work, you can try just chatting to him over the phone and then asking him again afterwards.
Then one you have him over just build up your relationship with him using your special options from Charismatic, Flirty and Hopeless Romantic until he becomes your 'Romantic Interest'.
** If you need more time to build the relationship, use the option to have him sleep over at your house, but you should build up the relationship quickly using the special traits. **