How do you get a SIM to be your Girlfriend on Sims 3?
Interact with the romantic interest and choose Romantic options (Flirt, Compliment Appearance, etc.) until their relationship bubble says So and So thinks you are being flirty. Interact with them, choose Romantic, then choose Propose Going Steady. If they accept, they will then be boyfriend/girlfriend.
How do you ask a Sim to be your Girlfriend?
Once two Sims have established some Romantic Relationship, you can ask to be Boyfriend/Girlfriend at just 30% (1/3 of the bar). This will not likely come up just walking up to them, but if you simply pull off a few Romantic socials, the option will appear.
How do you make Sims fall in love on Sims 3?
Open your Sims relationship menu and click on the relationship meter for the Sim you want to fall in love with. Drag the relationship bar over so that it is completely full. This will make the relationship between the two Sims strong enough to move on to the next step.