First and foremost before reaching the flirting scene you need to climb social ladders such as from friends to best friends. It will take a long time, energy and crazy flirting until you reach the level of dating. Then you will send request of dating until the other sim accepts it, after that you will go through several levels of relationships.

Instead of choosing friendly chat you can click on flirt (the pink double heart icon) and also you can pick other actions under flirt option for different ways of showing your affection.
1、Cheesy lines
2、Romantic Kiss
3、Flirty jokes
4、Confess Attraction
5、Compliment Appearance
6、Flatter
7、Swap numbers
8、Give Flowers

Source: http://www.bbgsite.com/socialgames/content/the-sims-social/guide/how-to-get-a-boyfriend-or-girlfriend.shtml

Is it really useful? I wanna to get a boyfriend!